Zilog Z800
Encyclopedia
The Zilog Z800 was a 16-bit
microprocessor
designed by Zilog
to be released in 1985. It was instruction compatible with their existing Z80
, and differed primarily in having on chip cache and MMU for a 16 MB address range, and also a huge number of new more orthogonal instructions and addressing modes. However Zilog essentially ignored the Z800 in favour of their 32-bit Z80000
and it appears the Z800 never entered mass production. The Z280 in 1987 was another try with almost the same design, but this time implemented in CMOS
.
set but the registers and instructions were significantly orthogonalized in order to make them more general purpose and powerful. Many new 8-bit and 16-bit operations were added, and the HL, IX, and IY registers were upgraded from their rather limited possibilities as accumulators in the Z80 to more versatile accumulators. In addition to the register operands possible in the Z80, they could be used with immediate data, direct address, register indirect, or indexed operands, even program counter-relative. Eight-bit operations had even more possibilities, including stack pointer-relative addressing and a choice of 8-bit or 16-bits immediate offsets.
The address bus was expanded to 24-bits to address 16 MB of memory. The chip was offered with either a 19-bit external bus for 512kB RAM, or a full 24-bit bus for 16MB RAM, the advantage to the smaller bus was a smaller 40-pin package. Like the Z80 before it, the Z800 retained the internal DRAM controller and clock, but added 256 bytes of RAM that could be used either as "scratchpad" RAM, or as a cache. When used in cache mode the programmer could configure it as a data or instruction cache, or both, and the internal memory controller then used it to reduce access to (slower) external memory.
There were also ambitious provisions for multiprocessing and either loosely or tightly coupled slave processors, with or without shared global memory. This was known as the extended processing architecture and extended processing units (EPU).
Another change was the addition of an optional 16-bit data bus, which doubled the rate at which it could access memory if set up properly. Combined with the two address bus sizes this meant that the chip was offered in a total of four versions:
, which is a less ambitious Z80 derivative that has had great success, probably because it is almost as simple to program and interface to as the original Z80.
A slightly different variant of the same design, the Z180, has been sold by Zilog for over twenty years, still maintained and developed.
) other attempts were made to extend the Z80 architecture, the 32-bit
Z380 (introduced 1994) was a commercial disappointment but is still in production for some specific telecom applications. On the other hand, the fast 24-bit
eZ80
(introduced 2001) has been both commercially successful and won engineering awards. It may be interesting to note that, unlike the Z800, Z280, and Z380, the eZ80
does not introduce many new instruction or addressing modes, but merely extends the 16-bit registers of the Z80 into 24 bits, enabling it to reach 256 times as much memory while its fully pipelined execution unit executes Z80 instructions four times as fast as the original.
16-bit
-16-bit architecture:The HP BPC, introduced in 1975, was the world's first 16-bit microprocessor. Prominent 16-bit processors include the PDP-11, Intel 8086, Intel 80286 and the WDC 65C816. The Intel 8088 was program-compatible with the Intel 8086, and was 16-bit in that its registers were 16...
microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
designed by Zilog
Zilog
Zilog, Inc., previously known as ZiLOG , is a manufacturer of 8-bit and 24-bit microcontrollers, and is most famous for its Intel 8080-compatible Z80 series.-History:...
to be released in 1985. It was instruction compatible with their existing Z80
Zilog Z80
The Zilog Z80 is an 8-bit microprocessor designed by Zilog and sold from July 1976 onwards. It was widely used both in desktop and embedded computer designs as well as for military purposes...
, and differed primarily in having on chip cache and MMU for a 16 MB address range, and also a huge number of new more orthogonal instructions and addressing modes. However Zilog essentially ignored the Z800 in favour of their 32-bit Z80000
Zilog Z80000
The Z80000 is Zilog's 32-bit processor from 1986, an expansion of its 16-bit predecessor, the Zilog Z8000. It includes multiprocessing capability, a six-stage instruction pipeline, and a 256-byte cache. Its memory addressing system can access 4 gigabytes of RAM. Described at the time as a...
and it appears the Z800 never entered mass production. The Z280 in 1987 was another try with almost the same design, but this time implemented in CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
.
Short description
There was no expansion of the registerProcessor register
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
set but the registers and instructions were significantly orthogonalized in order to make them more general purpose and powerful. Many new 8-bit and 16-bit operations were added, and the HL, IX, and IY registers were upgraded from their rather limited possibilities as accumulators in the Z80 to more versatile accumulators. In addition to the register operands possible in the Z80, they could be used with immediate data, direct address, register indirect, or indexed operands, even program counter-relative. Eight-bit operations had even more possibilities, including stack pointer-relative addressing and a choice of 8-bit or 16-bits immediate offsets.
The address bus was expanded to 24-bits to address 16 MB of memory. The chip was offered with either a 19-bit external bus for 512kB RAM, or a full 24-bit bus for 16MB RAM, the advantage to the smaller bus was a smaller 40-pin package. Like the Z80 before it, the Z800 retained the internal DRAM controller and clock, but added 256 bytes of RAM that could be used either as "scratchpad" RAM, or as a cache. When used in cache mode the programmer could configure it as a data or instruction cache, or both, and the internal memory controller then used it to reduce access to (slower) external memory.
There were also ambitious provisions for multiprocessing and either loosely or tightly coupled slave processors, with or without shared global memory. This was known as the extended processing architecture and extended processing units (EPU).
Another change was the addition of an optional 16-bit data bus, which doubled the rate at which it could access memory if set up properly. Combined with the two address bus sizes this meant that the chip was offered in a total of four versions:
| part # | # of pins | data bus | address bus |
|---|---|---|---|
| Z8108 | 40 | 8-bit | 19-bit (512kB) |
| Z8116 | 40 | 16-bit | 19-bit (512kB) |
| Z8208 | 64 | 8-bit | 24-bit (16MB) |
| Z8216 | 64 | 16-bit | 24-bit (16MB) |
Reason for the failure
The Z800 was, in most ways, a mini computer inspired "super Z80" that would run existing, and larger, programs at considerably higher speeds. However the address and data buses were multiplexed and the chip was, also in other respects, somewhat complicated to program and interface to. Calculation of exact execution times was also very much harder to do than for the Z80. Moreover, the plain Z80 were good enough for most applications at the time so the extra computing power was, in many cases, not worth the added complexity. Bad marketing seems to have hurt the product as well. Hitachi later developed the HD64180HD64180
The HD64180 is a Z80-based embedded microprocessor developed by Hitachi with an integrated memory management unit . The Hitachi HD64180 "Super Z80" was later licensed to Zilog and sold by them as the Z64180 and with some enhancements as the Zilog Z180. It has the following features:* Memory...
, which is a less ambitious Z80 derivative that has had great success, probably because it is almost as simple to program and interface to as the original Z80.
A slightly different variant of the same design, the Z180, has been sold by Zilog for over twenty years, still maintained and developed.
More successful Z80 derivatives (from Zilog)
Apart from the successful Z180 (developed largely by HitachiHitachi, Ltd.
is a Japanese multinational conglomerate headquartered in Marunouchi 1-chome, Chiyoda, Tokyo, Japan. The company is the parent of the Hitachi Group as part of the larger DKB Group companies...
) other attempts were made to extend the Z80 architecture, the 32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
Z380 (introduced 1994) was a commercial disappointment but is still in production for some specific telecom applications. On the other hand, the fast 24-bit
24-bit
Notable 24-bit machines include the ICT 1900 series and the Harris H series.The IBM System/360, announced in 1964, was a popular computer system with 24-bit addressing and 32-bit general registers and arithmetic...
eZ80
Zilog eZ80
The Zilog eZ80 is an 8-bit microprocessor which is essentially an updated version of the company's earlier Z80 8-bit microprocessor.The eZ80 is binary compatible with the Z80 and Z180, but almost four times as fast as the original Z80 chip at the same clock frequency...
(introduced 2001) has been both commercially successful and won engineering awards. It may be interesting to note that, unlike the Z800, Z280, and Z380, the eZ80
Zilog eZ80
The Zilog eZ80 is an 8-bit microprocessor which is essentially an updated version of the company's earlier Z80 8-bit microprocessor.The eZ80 is binary compatible with the Z80 and Z180, but almost four times as fast as the original Z80 chip at the same clock frequency...
does not introduce many new instruction or addressing modes, but merely extends the 16-bit registers of the Z80 into 24 bits, enabling it to reach 256 times as much memory while its fully pipelined execution unit executes Z80 instructions four times as fast as the original.