Apple II graphics
Encyclopedia
The Apple II graphics were composed of idiosyncratic modes and settings that could be exploited. This graphics system
debuted on the original Apple II, continued with the Apple II Plus
and was carried forward and expanded with the Apple IIe
, Enhanced IIe, IIc
, IIc Plus and IIGS
.
's chip
-saving design. Many home computer systems of the time (as well as today's PC-compatible machines
) had architecture which assigned consecutive blocks of memory to non-consecutive rows on the screen in graphic modes, i.e., interleaving
. But instead of the usual 2:1 factor, Apple's text and graphics modes are based on interleave factors of 8:1 and 64:1.
A second peculiarity of Apple II graphics—the so-called "color fringes"—is yet another by-product of Wozniak's design. While these occur in all modes, they play a crucial role in Hi-Resolution or Hi-Res mode (see below).
providing a rough NTSC
, PAL
, or SECAM
composite video
output. This enabled the computer to be connected to any composite video monitor conforming to the same standard for which the machine was configured. However the quality of this output was sketchy; the sync signalling was close enough for monitors—which are fairly forgiving—but did not conform closely enough to standards to be suitable for broadcast applications, or even input to a video recorder, without intervening processing. (The exception was the Extended Back version of the Bell & Howell branded black II Plus, which did provide proper video sync, as well as other media oriented features.)
In addition to the composite video output jack, the IIc, IIc Plus, and the IIGS featured a two-row, 15-pin output. In the IIc and IIc Plus, this connector was a special-purpose video connector for adapters to digital RGB monitors and RF modulator
s. In the IIGS it was an output for an analog RGB monitor specially designed for the IIGS.
monitor through a DE9
output jack.
with the colorburst
signal to be interpreted as color by a composite video display.
High resolution provides two pixels per colorburst cycle, allowing for two possible colors if one pixel is on, black if no pixels are on, or white if both pixels are on. By shifting the alignment of the pixels to the colorburst signal by 90°, two more colors can be displayed for a total of four possible colors. Low resolution allows for four bit
s per cycle, but repeats the bit pattern several times per low resolution pixel. Double high resolution also displays four pixels per cycle. (Again, a 90° phase shift would double the colors available, but is not supported in double high resolution mode). See the sections below for more details.
There are 16 colors available for use in this mode (actually 15, since the two shades of gray are almost identical). Note that six of the colors are identical to the colors available in High-Resolution (Hi-Res) mode.
The colors were created by filling the pixel with a repeating 4-bit binary pattern in such a manner that each bit group fit within one cycle of the colorburst
reference signal. Color displays would interpret this pattern as a color signal. On monochrome monitors, or if the colorburst signal was turned off, the display would reveal these bit patterns. There are two equivalent grey shades as 5 (0101) is equivalent to 10 (1010) based on how the colors mix together; the "on" bits a polar opposites of each other on the quadrature
color signal, so they cancel each other and display as grey.
This mode is mapped to the same area of memory as the main 40-column text screen (0x400 through 0x7FF), with each byte storing two pixels one on top of the other.
The Lo-Res graphics mode offered built-in commands to clear the screen, change the drawing color, plot individual pixels, plot horizontal lines, and plot vertical lines. There was also a "SCRN" function to extract the color stored in any pixel, one sorely lacking in the other modes.
each, with a remainder of eight bytes left after the third row is stored. But these bytes are not left empty. Instead, they are used variously by motherboard firmware and expansion card firmware to store important information, mostly about external devices attached to the computer. This created problems when the user loaded a text or a lo-res graphics screen directly into video memory—replacing the current information in the holes with what was there at save-time. (Disk head recalibration was a common side-effect, when the disk controller found its memory—in a screen hole—of where the head was, suddenly not to match the header data of the track that it was reading). The programmers at Apple responded by programming ProDOS
so the user could not directly load a file (screen data, or otherwise) into 0x400-0x7FF. ProDOS programs to properly load data to this portion of memory soon arose; several appeared in Nibble magazine
.
programs are loaded at 801h (2049) by default; therefore, they will occupy the Text Screen 2 space unless the computer is instructed to load a program elsewhere in memory. By contrast, some commercial software programs for the Apple II used this memory space for various purposes — usually to display a help screen.
Unlike the other Apple II machine types, the Apple IIGS featured a processor (the 65816) which could address more than 64K of RAM
without special tricks. In the IIgs, RAM was demarcated into banks of 64K. For example, bank 0xE0 consisted of the range 0xE00000 through 0xE0FFFF. The Apple IIgs had a chip called the "Mega II" which allowed it to run most programs written for other Apple II computers. The IIgs architecture
mapped the screen data to memory bank 0xE0. However, in IIe emulation mode, screen data was stored in bank 0x00. This presented a problem. The designers of the Mega II included routines to copy most screen data to bank 0xE0 to ensure that Apple IIe-specific programs worked properly. But they forgot about Text Screen 2. This was not discovered until the Mega II chips had made it into the IIgs machines. So the firmware designers added a CDA (classic desk accessory—accessible from the IIgs Desk Accessories menu, invoked with Apple-Option-Escape) called "Alternate Display Mode" , which, at the expense of a little bit of CPU time, performed the task for the few programs that needed it. It could be turned on and off at whim, but reverted to off upon resetting the computer.
, either one could be initialized, using the commands HGR for the first screen or HGR2 for the second.
The Applesoft BASIC ROM contained routines to clear either of two Hi-Res screens, draw lines and points, and set the drawing color. The ROM also contained routines to draw, erase, scale and rotate vector
-based shapes. Oddly enough, there were no routines to plot bitmapped shapes
, draw circles and arcs
, or fill a drawn area. Fortunately, many programs were written; many appeared in Nibble and other Apple II magazines.
The user could "switch in" four lines of text in the Hi-Res mode, just like in Lo-Res mode; however, this hid the bottom 32 lines, resulting in a 280x160 picture. (The ROM routines could still modify the bottom, even though it was hidden.)
The Apple II's Hi-Res mode was peculiar even by the standards of the day. While the CGA
card on the competing IBM PC
allowed the user to select one of two color sets for creating 320×200 graphics, only four colors (the background color and three drawing colors) were available at a time. By contrast, the Apple offered eight colors for high-resolution graphics (actually six, since black and white were both repeated in the scheme). There was a catch, however. Each row of 280 pixels was broken up into 40 blocks of seven pixels each. In memory, the lower seven bits of each byte represented the pixels, while the most significant bit
served a special purpose. It determined which colors to display onscreen.
While this feature allows six colors onscreen simultaneously, it does have one unpleasant side effect. For example, if a programmer tried to draw a blue line on top of a green one, portions of the green line would change to orange. This is because drawing the blue line sets the MSB for each block of seven pixels in this case. "Green" and "orange" pixels are represented the same way in memory; the difference is in the setting (or clearing) of the MSB. Another side effect is that drawing a pixel required dividing by seven.
The Hi-Res mode on the Apple II was also peculiar for its 64:1 interleave factor. This was a direct result of Steve Wozniak's chip-saving design The 64:1 factor resulted in a "Venetian blind" effect when loading a Hi-Res screen into memory from floppy disk
(or sometimes RAM disk
) with the soft switches already set. "Screen holes" occur in the Hi-Res mode just as they do in the Lo-Res and text modes. Nothing was usually stored there—though they were occasionally used to store code in self-displaying executable pictures. Another notable exception is the Fotofile (FOT) format inherited by ProDOS
from Apple SOS, which included metadata
in the 121st byte (the first byte of the first hole) indicating how it should be displayed (colour mode, resolution), or converted to other graphics formats.
Finally, another quirk of Wozniak's design is that while any pixel could be black or white, only pixels with odd X-coordinates could be green or orange. Likewise, only even-numbered pixels could be violet or blue. This is where the so-called "fringe benefit" comes in. The Apple video hardware interprets a sequence of three or more turned-on horizontal pixels as solid white, while a sequence of alternating pixels would display as color. Similarly, a sequence of three or more turned-off horizontal pixels would display as black.
There was no built-in command to extract the color of a pixel on the Hi-Res screen, or even to determine whether it was on at all. Several programs to determine if a pixel was lit were written, and a program to extract the pixel's true color was published in the April 1990 edition of Nibble.
The Apple II Hi-Res graphics mode did have one crucial advantage over IBM's CGA. Just as there are two text screen pages (and two Lo-Res graphics pages), so there are also two Hi-Res pages, mapped one right after the other in memory. (The second Hi-Res screen was mapped to 0x4000-0x5FFF, or 16384-24575 in decimal.) The CGA, on the other hand, supported only one graphics page at a time. Not until the EGA
video card was released could the IBM platform support multiple pages of graphics simultaneously. Naturally, this simplified animation on the Apple II. A programmer could display one page while altering the other (hidden) page.
If you were creative, you could press Open-Apple & Reset and set soft switches to capture images from games and other programs. Favorite scenes could be recorded from games in this manner. The only drawback is a bit of noise which appears as 2 to 3 short lines of white lines appearing in the image. These can be edited out using a paint package. It was possible to BSAVE these images to a floppy and create a slide show or a static image, because a Soft-Reset did not clear the video memory on Hi-Res images.
10 PRINT CHR$(4)"PR#3" : PRINT CHR$(0); : POKE 49246,0 : GR
(Note that PR#3 is deferred to the operating system, with PRINT CHR$(4) to avoid disconnecting it from BASIC—for complicated reasons. This is followed by a PRINT command to send a null character, because the newly assigned output device doesn't get initialised until the first character is sent to it—a common source of confusion.)
Once this was done, the Double Lo-Res screen was displayed and cleared, and the PLOT, HLIN, and VLIN commands worked normally with the x coordinate range extended to 0 though 79. (Only the Apple IIc and IIgs supported this in firmware. Using double-lo-res mode from BASIC on a IIe was much more complicated without adding an & command extension to BASIC.)
There were two major problems when using this mode in Applesoft. First, once the mode was activated, access to the printer became complicated, due to the 80 column display firmware being handled like a printer. Second, the SCRN (pixel read) function did not work properly. Fortunately, there was a program in the March 1990 issue of Nibble that took care of this problem.
At least one commercially available BASIC
compiler
, ZBASIC
from Zedcor Systems, was known to support Double Lo-Res graphics.
The ProDOS implementation of its RAM disk
made access to the Double Hi-Res screen easier by making the first 8 KB file saved to /RAM store its data at 0x012000 to 0x013fff by design. Also, a second page was possible, and a second file (or a larger first file) would store its data at 0x014000 to 0x015fff. However, access via the ProDOS file system was slow and not well suited to page-flipping animation in Double Hi-Res, beyond the memory requirements.
Despite the complexities involved in programming
and using this mode, there were numerous applications which made use of it. Double Hi-Res graphics were featured in business applications, educational software, and games alike. The Apple version of GEOS
used Double Hi-Res, as did Brøderbund
's famous paint program, DazzleDraw. Numerous arcade games, and games written for other computers, were ported
to the Apple II platform, and many took advantage of this graphics mode. There were also numerous utility programs and plug-in printer cards that allowed the user to print Double Hi-Res graphics on a dot-matrix printer or even the LaserWriter
.
The Apple IIGS featured not only the graphics modes of its precursors, but several new modes similar to ones found on the Atari ST
and Commodore Amiga.
Computer graphics
Computer graphics are graphics created using computers and, more generally, the representation and manipulation of image data by a computer with help from specialized software and hardware....
debuted on the original Apple II, continued with the Apple II Plus
Apple II Plus
The Apple II Plus was the second model of the Apple II series of personal computers produced by Apple Computer, Inc. It was sold new from June 1979 to December 1982.-Features:...
and was carried forward and expanded with the Apple IIe
Apple IIe
The Apple IIe is the third model in the Apple II series of personal computers produced by Apple Computer. The e in the name stands for enhanced, referring to the fact that several popular features were now built-in that were only available as upgrades and add-ons in earlier models...
, Enhanced IIe, IIc
Apple IIc
The Apple IIc, the fourth model in the Apple II series of personal computers, was Apple Computer’s first endeavor to produce a portable computer. The end result was a notebook-sized version of the Apple II that could be transported from place to place...
, IIc Plus and IIGS
Apple IIGS
The Apple , the fifth and most powerful model in the Apple II series of personal computers produced by Apple Computer. The "GS" in the name stands for Graphics and Sound, referring to its enhanced graphics and sound capabilities, both of which greatly surpassed previous models of the line...
.
Peculiarity of graphics modes
The graphic modes of the Apple II series were peculiar even by the standards of the late 1970s and early 1980s. One notable peculiarity of these modes is a direct result of Apple founder Steve WozniakSteve Wozniak
Stephen Gary "Woz" Wozniak is an American computer engineer and programmer who founded Apple Computer, Co. with Steve Jobs and Ronald Wayne...
's chip
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
-saving design. Many home computer systems of the time (as well as today's PC-compatible machines
IBM PC compatible
IBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to...
) had architecture which assigned consecutive blocks of memory to non-consecutive rows on the screen in graphic modes, i.e., interleaving
Interleaving
In computer science and telecommunication, interleaving is a way to arrange data in a non-contiguous way to increase performance.It is typically used:* In error-correction coding, particularly within data transmission, disk storage, and computer memory....
. But instead of the usual 2:1 factor, Apple's text and graphics modes are based on interleave factors of 8:1 and 64:1.
A second peculiarity of Apple II graphics—the so-called "color fringes"—is yet another by-product of Wozniak's design. While these occur in all modes, they play a crucial role in Hi-Resolution or Hi-Res mode (see below).
Video output on the machines
Reading or writing to certain memory addresses controlled "soft switches", which allowed the user to do many different things including displaying the graphics screen (any type) without erasing it, displaying the text screen, clearing the last key pressed, or accessing different memory banks. For example, one could switch from mixed graphics and text to an all-graphics display by accessing location 0xC052 (49234). Then, to go back to mixed graphics and text, one would access 0xC053 (49235).Built-in video output hardware
All Apple II machines featured an RCA jackRCA connector
An RCA connector, sometimes called a phono connector or cinch connector, is a type of electrical connector commonly used to carry audio and video signals...
providing a rough NTSC
NTSC
NTSC, named for the National Television System Committee, is the analog television system that is used in most of North America, most of South America , Burma, South Korea, Taiwan, Japan, the Philippines, and some Pacific island nations and territories .Most countries using the NTSC standard, as...
, PAL
PAL
PAL, short for Phase Alternating Line, is an analogue television colour encoding system used in broadcast television systems in many countries. Other common analogue television systems are NTSC and SECAM. This page primarily discusses the PAL colour encoding system...
, or SECAM
SECAM
SECAM, also written SÉCAM , is an analog color television system first used in France....
composite video
Composite video
Composite video is the format of an analog television signal before it is combined with a sound signal and modulated onto an RF carrier. In contrast to component video it contains all required video information, including colors in a single line-level signal...
output. This enabled the computer to be connected to any composite video monitor conforming to the same standard for which the machine was configured. However the quality of this output was sketchy; the sync signalling was close enough for monitors—which are fairly forgiving—but did not conform closely enough to standards to be suitable for broadcast applications, or even input to a video recorder, without intervening processing. (The exception was the Extended Back version of the Bell & Howell branded black II Plus, which did provide proper video sync, as well as other media oriented features.)
In addition to the composite video output jack, the IIc, IIc Plus, and the IIGS featured a two-row, 15-pin output. In the IIc and IIc Plus, this connector was a special-purpose video connector for adapters to digital RGB monitors and RF modulator
RF modulator
An RF modulator is a device that takes a baseband input signal and outputs a radio frequency-modulated signal....
s. In the IIGS it was an output for an analog RGB monitor specially designed for the IIGS.
Add-on video output cards
Numerous add-on video display cards were available for the Apple II series. Some simply added 80-column and lowercase display capabilities, while others allowed output to an IBM CGAColor Graphics Adapter
The Color Graphics Adapter , originally also called the Color/Graphics Adapter or IBM Color/Graphics Monitor Adapter, introduced in 1981, was IBM's first color graphics card, and the first color computer display standard for the IBM PC....
monitor through a DE9
D-subminiature
The D-subminiature or D-sub is a common type of electrical connector. They are named for their characteristic D-shaped metal shield. When they were introduced, D-subs were among the smaller connectors used on computer systems....
output jack.
Color on the Apple II
The Apple II video output is really a monochrome display based upon the bit patterns in the video memory (or pixels). These pixels are combined in quadratureQuadrature
Quadrature may refer to:In signal processing:*Quadrature amplitude modulation , a modulation method of using both an carrier wave and a 'quadrature' carrier wave that is 90° out of phase with the main, or in-phase, carrier...
with the colorburst
Colorburst
Colorburst is a analog video, composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal...
signal to be interpreted as color by a composite video display.
High resolution provides two pixels per colorburst cycle, allowing for two possible colors if one pixel is on, black if no pixels are on, or white if both pixels are on. By shifting the alignment of the pixels to the colorburst signal by 90°, two more colors can be displayed for a total of four possible colors. Low resolution allows for four bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
s per cycle, but repeats the bit pattern several times per low resolution pixel. Double high resolution also displays four pixels per cycle. (Again, a 90° phase shift would double the colors available, but is not supported in double high resolution mode). See the sections below for more details.
Low-Resolution (Lo-Res) graphics
The Lo-Res graphics mode (often known as GR after the BASIC command) was 40 pixels wide, corresponding to the 40 columns on the normal Apple II text screen. This mode could display either 40 rows of pixels with four lines of text at the bottom of the screen, or 48 rows of pixels with no text. The default for this was 40×40 graphics with text.There are 16 colors available for use in this mode (actually 15, since the two shades of gray are almost identical). Note that six of the colors are identical to the colors available in High-Resolution (Hi-Res) mode.
The colors were created by filling the pixel with a repeating 4-bit binary pattern in such a manner that each bit group fit within one cycle of the colorburst
Colorburst
Colorburst is a analog video, composite video signal generated by a video-signal generator used to keep the chrominance subcarrier synchronized in a color television signal...
reference signal. Color displays would interpret this pattern as a color signal. On monochrome monitors, or if the colorburst signal was turned off, the display would reveal these bit patterns. There are two equivalent grey shades as 5 (0101) is equivalent to 10 (1010) based on how the colors mix together; the "on" bits a polar opposites of each other on the quadrature
Quadrature
Quadrature may refer to:In signal processing:*Quadrature amplitude modulation , a modulation method of using both an carrier wave and a 'quadrature' carrier wave that is 90° out of phase with the main, or in-phase, carrier...
color signal, so they cancel each other and display as grey.
This mode is mapped to the same area of memory as the main 40-column text screen (0x400 through 0x7FF), with each byte storing two pixels one on top of the other.
The Lo-Res graphics mode offered built-in commands to clear the screen, change the drawing color, plot individual pixels, plot horizontal lines, and plot vertical lines. There was also a "SCRN" function to extract the color stored in any pixel, one sorely lacking in the other modes.
The Lo-res "screen holes"
A block of 128 bytes stores three rows of 40 charactersCharacter (computing)
In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language....
each, with a remainder of eight bytes left after the third row is stored. But these bytes are not left empty. Instead, they are used variously by motherboard firmware and expansion card firmware to store important information, mostly about external devices attached to the computer. This created problems when the user loaded a text or a lo-res graphics screen directly into video memory—replacing the current information in the holes with what was there at save-time. (Disk head recalibration was a common side-effect, when the disk controller found its memory—in a screen hole—of where the head was, suddenly not to match the header data of the track that it was reading). The programmers at Apple responded by programming ProDOS
ProDOS
ProDOS was the name of two similar operating systems for the Apple II series of personal computers. The original ProDOS, renamed ProDOS 8 in version 1.2, was the last official operating system usable by all Apple II series computers, and was distributed from 1983 to 1993...
so the user could not directly load a file (screen data, or otherwise) into 0x400-0x7FF. ProDOS programs to properly load data to this portion of memory soon arose; several appeared in Nibble magazine
Nibble (magazine)
Nibble was a magazine for Apple II computer users with a focus on hobbyist programming. The name meant "half a byte" or "four bits." Its slogan was "The Magazine for Apple II Enthusiasts." Most of the articles incorporated the source code of a small to medium-sized utility, application program,...
.
Screen 2 Low-Resolution graphics and text
Having two screens for displaying video images was an integral part of the Apple II family design. Accessing memory location 0xC055 (49237) displayed "Screen 2" regardless of how the other "soft switches" were set. The text and Lo-Res Screen 2 space ranged from 0x800 (2048) to 0xBFF (3071). The interleaving is exactly the same as for the main screen ("Screen 1"). Applesoft BASICApplesoft BASIC
Applesoft BASIC was a dialect of Microsoft BASIC supplied with the Apple II series of computers. It superseded Integer BASIC and was the BASIC in ROM in all Apple II series computers after the original Apple II model. It was also referred to as FP because of the command used to invoke it instead...
programs are loaded at 801h (2049) by default; therefore, they will occupy the Text Screen 2 space unless the computer is instructed to load a program elsewhere in memory. By contrast, some commercial software programs for the Apple II used this memory space for various purposes — usually to display a help screen.
"Alternate Display Mode" on the Apple IIGS
Unlike the other Apple II machine types, the Apple IIGS featured a processor (the 65816) which could address more than 64K of RAM
Ram
-Animals:*Ram, an uncastrated male sheep*Ram cichlid, a species of freshwater fish endemic to Colombia and Venezuela-Military:*Battering ram*Ramming, a military tactic in which one vehicle runs into another...
without special tricks. In the IIgs, RAM was demarcated into banks of 64K. For example, bank 0xE0 consisted of the range 0xE00000 through 0xE0FFFF. The Apple IIgs had a chip called the "Mega II" which allowed it to run most programs written for other Apple II computers. The IIgs architecture
Computer architecture
In computer science and engineering, computer architecture is the practical art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals and the formal modelling of those systems....
mapped the screen data to memory bank 0xE0. However, in IIe emulation mode, screen data was stored in bank 0x00. This presented a problem. The designers of the Mega II included routines to copy most screen data to bank 0xE0 to ensure that Apple IIe-specific programs worked properly. But they forgot about Text Screen 2. This was not discovered until the Mega II chips had made it into the IIgs machines. So the firmware designers added a CDA (classic desk accessory—accessible from the IIgs Desk Accessories menu, invoked with Apple-Option-Escape) called "Alternate Display Mode" , which, at the expense of a little bit of CPU time, performed the task for the few programs that needed it. It could be turned on and off at whim, but reverted to off upon resetting the computer.
High-Resolution (Hi-Res) graphics
When the Apple II came out, a new mode had been added for 280×192 high-resolution graphics. Like Lo-Res mode, hi-res mode had two screens; in Applesoft BASICApplesoft BASIC
Applesoft BASIC was a dialect of Microsoft BASIC supplied with the Apple II series of computers. It superseded Integer BASIC and was the BASIC in ROM in all Apple II series computers after the original Apple II model. It was also referred to as FP because of the command used to invoke it instead...
, either one could be initialized, using the commands HGR for the first screen or HGR2 for the second.
The Applesoft BASIC ROM contained routines to clear either of two Hi-Res screens, draw lines and points, and set the drawing color. The ROM also contained routines to draw, erase, scale and rotate vector
Vector graphics
Vector graphics is the use of geometrical primitives such as points, lines, curves, and shapes or polygon, which are all based on mathematical expressions, to represent images in computer graphics...
-based shapes. Oddly enough, there were no routines to plot bitmapped shapes
Raster graphics
In computer graphics, a raster graphics image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium...
, draw circles and arcs
Arc (geometry)
In geometry, an arc is a closed segment of a differentiable curve in the two-dimensional plane; for example, a circular arc is a segment of the circumference of a circle...
, or fill a drawn area. Fortunately, many programs were written; many appeared in Nibble and other Apple II magazines.
The user could "switch in" four lines of text in the Hi-Res mode, just like in Lo-Res mode; however, this hid the bottom 32 lines, resulting in a 280x160 picture. (The ROM routines could still modify the bottom, even though it was hidden.)
The Apple II's Hi-Res mode was peculiar even by the standards of the day. While the CGA
Color Graphics Adapter
The Color Graphics Adapter , originally also called the Color/Graphics Adapter or IBM Color/Graphics Monitor Adapter, introduced in 1981, was IBM's first color graphics card, and the first color computer display standard for the IBM PC....
card on the competing IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
allowed the user to select one of two color sets for creating 320×200 graphics, only four colors (the background color and three drawing colors) were available at a time. By contrast, the Apple offered eight colors for high-resolution graphics (actually six, since black and white were both repeated in the scheme). There was a catch, however. Each row of 280 pixels was broken up into 40 blocks of seven pixels each. In memory, the lower seven bits of each byte represented the pixels, while the most significant bit
Most significant bit
In computing, the most significant bit is the bit position in a binary number having the greatest value...
served a special purpose. It determined which colors to display onscreen.
While this feature allows six colors onscreen simultaneously, it does have one unpleasant side effect. For example, if a programmer tried to draw a blue line on top of a green one, portions of the green line would change to orange. This is because drawing the blue line sets the MSB for each block of seven pixels in this case. "Green" and "orange" pixels are represented the same way in memory; the difference is in the setting (or clearing) of the MSB. Another side effect is that drawing a pixel required dividing by seven.
The Hi-Res mode on the Apple II was also peculiar for its 64:1 interleave factor. This was a direct result of Steve Wozniak's chip-saving design The 64:1 factor resulted in a "Venetian blind" effect when loading a Hi-Res screen into memory from floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
(or sometimes RAM disk
RAM disk
A RAM disk or RAM drive is a block of RAM that a computer's software is treating as if the memory were a disk drive...
) with the soft switches already set. "Screen holes" occur in the Hi-Res mode just as they do in the Lo-Res and text modes. Nothing was usually stored there—though they were occasionally used to store code in self-displaying executable pictures. Another notable exception is the Fotofile (FOT) format inherited by ProDOS
ProDOS
ProDOS was the name of two similar operating systems for the Apple II series of personal computers. The original ProDOS, renamed ProDOS 8 in version 1.2, was the last official operating system usable by all Apple II series computers, and was distributed from 1983 to 1993...
from Apple SOS, which included metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
in the 121st byte (the first byte of the first hole) indicating how it should be displayed (colour mode, resolution), or converted to other graphics formats.
Finally, another quirk of Wozniak's design is that while any pixel could be black or white, only pixels with odd X-coordinates could be green or orange. Likewise, only even-numbered pixels could be violet or blue. This is where the so-called "fringe benefit" comes in. The Apple video hardware interprets a sequence of three or more turned-on horizontal pixels as solid white, while a sequence of alternating pixels would display as color. Similarly, a sequence of three or more turned-off horizontal pixels would display as black.
There was no built-in command to extract the color of a pixel on the Hi-Res screen, or even to determine whether it was on at all. Several programs to determine if a pixel was lit were written, and a program to extract the pixel's true color was published in the April 1990 edition of Nibble.
The Apple II Hi-Res graphics mode did have one crucial advantage over IBM's CGA. Just as there are two text screen pages (and two Lo-Res graphics pages), so there are also two Hi-Res pages, mapped one right after the other in memory. (The second Hi-Res screen was mapped to 0x4000-0x5FFF, or 16384-24575 in decimal.) The CGA, on the other hand, supported only one graphics page at a time. Not until the EGA
EGA
EGA may stand for*EGA: estimated gestational age, medical*Eagle, Globe, and Anchor, the emblem of the United States Marine Corps*Egyptian German Automotive Company, the Mercedes-Benz manufacturer in Egypt*Elegant Gothic Aristocrat, a fashion line...
video card was released could the IBM platform support multiple pages of graphics simultaneously. Naturally, this simplified animation on the Apple II. A programmer could display one page while altering the other (hidden) page.
If you were creative, you could press Open-Apple & Reset and set soft switches to capture images from games and other programs. Favorite scenes could be recorded from games in this manner. The only drawback is a bit of noise which appears as 2 to 3 short lines of white lines appearing in the image. These can be edited out using a paint package. It was possible to BSAVE these images to a floppy and create a slide show or a static image, because a Soft-Reset did not clear the video memory on Hi-Res images.
Graphic modes on later models (IIe, IIc, IIc Plus, IIGS)
Soon after the introduction of the Apple IIe, the Apple engineers realised that the video bandwidth doubling circuitry used to implement 80-column text mode could be easily extended to include the machines graphics modes. Since the signal was present at the auxiliary slot connector which housed the "Extended 80 Column Card, Annunciator 3 on the game port was overloaded to activate double resolution graphics when both 80 column video and a graphics mode was selected. Replacement motherboards (called the Revision B motherboard) were offered free of charge to owners of the Apple IIe to upgrade their machines with double resolution graphics capabilities. For this reason, machines with the original Revision A motherboard are extremely rare. Subsequent Apple II models also implement the double resolution graphics modes.Double Low-Resolution
This was an 80×40 (or 80×48) graphics mode available only on 80-column machines. Under Applesoft BASIC, enabling this mode required three steps. First, enabling 80 column mode with PR#3, Then enabling double-density graphics with POKE 49246,0, followed by GR.10 PRINT CHR$(4)"PR#3" : PRINT CHR$(0); : POKE 49246,0 : GR
(Note that PR#3 is deferred to the operating system, with PRINT CHR$(4) to avoid disconnecting it from BASIC—for complicated reasons. This is followed by a PRINT command to send a null character, because the newly assigned output device doesn't get initialised until the first character is sent to it—a common source of confusion.)
Once this was done, the Double Lo-Res screen was displayed and cleared, and the PLOT, HLIN, and VLIN commands worked normally with the x coordinate range extended to 0 though 79. (Only the Apple IIc and IIgs supported this in firmware. Using double-lo-res mode from BASIC on a IIe was much more complicated without adding an & command extension to BASIC.)
There were two major problems when using this mode in Applesoft. First, once the mode was activated, access to the printer became complicated, due to the 80 column display firmware being handled like a printer. Second, the SCRN (pixel read) function did not work properly. Fortunately, there was a program in the March 1990 issue of Nibble that took care of this problem.
At least one commercially available BASIC
BASIC
BASIC is a family of general-purpose, high-level programming languages whose design philosophy emphasizes ease of use - the name is an acronym from Beginner's All-purpose Symbolic Instruction Code....
compiler
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
, ZBASIC
ZBasic
ZBasic was a compiler first released by Simutek in 1980. The combined efforts of Andrew Gariepy, Scott Terry, David Overton, Greg Branche, and Halbert Laing lead to versions for MS-DOS, Apple II, Macintosh, CP/M and TRS-80 computers. ZBasic was a very fast, efficient and quite advanced BASIC...
from Zedcor Systems, was known to support Double Lo-Res graphics.
Double High-Resolution
The composition of the Double Hi-Res screen is very complicated. In addition to the 64:1 interleaving, the pixels in the individual rows are stored in an unusual way: each pixel was half its usual width and each byte of pixels alternated between the first and second bank of 64KB memory. Where three consecutive on pixels were white, six were now required in double high-resolution. Effectively, all pixel patterns used to make color in Lo-Res graphics blocks could be reproduced in Double Hi-Res graphics.The ProDOS implementation of its RAM disk
RAM disk
A RAM disk or RAM drive is a block of RAM that a computer's software is treating as if the memory were a disk drive...
made access to the Double Hi-Res screen easier by making the first 8 KB file saved to /RAM store its data at 0x012000 to 0x013fff by design. Also, a second page was possible, and a second file (or a larger first file) would store its data at 0x014000 to 0x015fff. However, access via the ProDOS file system was slow and not well suited to page-flipping animation in Double Hi-Res, beyond the memory requirements.
Applications using Double High-Resolution
Despite the complexities involved in programming
Computer programming
Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
and using this mode, there were numerous applications which made use of it. Double Hi-Res graphics were featured in business applications, educational software, and games alike. The Apple version of GEOS
GEOS
GEOS may refer to:Computer software*GEOS , an operating system originally designed for the Commodore 64*GEOS , a DOS-based graphical user interface and x86 operating system...
used Double Hi-Res, as did Brøderbund
Brøderbund
Brøderbund Software, Inc. was an American maker of computer games, educational software and The Print Shop productivity tools. It was best known as the original creator and publisher of the popular Carmen Sandiego games. The company was founded in Eugene, Oregon, but moved to San Rafael,...
's famous paint program, DazzleDraw. Numerous arcade games, and games written for other computers, were ported
Porting
In computer science, porting is the process of adapting software so that an executable program can be created for a computing environment that is different from the one for which it was originally designed...
to the Apple II platform, and many took advantage of this graphics mode. There were also numerous utility programs and plug-in printer cards that allowed the user to print Double Hi-Res graphics on a dot-matrix printer or even the LaserWriter
LaserWriter
The LaserWriter was a laser printer with built-in PostScript interpreter introduced by Apple in 1985. It was one of the first laser printers available to the mass market...
.
Apple IIGS graphics modes
See also: Apple IIGS graphics modesThe Apple IIGS featured not only the graphics modes of its precursors, but several new modes similar to ones found on the Atari ST
Atari ST
The Atari ST is a home/personal computer that was released by Atari Corporation in 1985 and commercially available from that summer into the early 1990s. The "ST" officially stands for "Sixteen/Thirty-two", which referred to the Motorola 68000's 16-bit external bus and 32-bit internals...
and Commodore Amiga.
See also
- Apple IIApple IIThe Apple II is an 8-bit home computer, one of the first highly successful mass-produced microcomputer products, designed primarily by Steve Wozniak, manufactured by Apple Computer and introduced in 1977...
- Apple IIc PlusApple IIc PlusThe Apple IIc Plus is the sixth and final model in the Apple II line of personal computers, produced by Apple Computer. The "Plus" in the name was a reference to the additional features it offered over the original portable Apple IIc, such as greater storage capacity , increased processing speed,...
- Apple IIe CardApple IIe CardThe Apple IIe Card is a compatibility card which allows compatible Macs to run software designed for Apple II computers...
- Nibble magazineNibble (magazine)Nibble was a magazine for Apple II computer users with a focus on hobbyist programming. The name meant "half a byte" or "four bits." Its slogan was "The Magazine for Apple II Enthusiasts." Most of the articles incorporated the source code of a small to medium-sized utility, application program,...
- ComputerEyes