Atari MMU
Encyclopedia
The Atari MMU is a custom MMU
chip for the Atari 8-bit computers. It enables access to the hardware registers on ANTIC
, GTIA, POKEY
and 6520 PIA.
The later XL/XE MMU (C061618) also selects OS
ROM
, Atari BASIC
ROM, self-test ROM and LEDs in the 1200XL.
The user cannot directly manipulate the Atari MMU, but selects the various ROMS and memory via the memory-mapped hardware register known as PORTB (5401710 or D30116). Atari changed PORTB from an input port on the 400/800 machines to an output port on the XL/XE machines, leaving two joystick ports instead of four on the XL/XE machines.
By setting and clearing specific bits in PORTB, the user can access either the ROMs or memory locations. No synchronization is required as the OS handles the access.
The bit assignments for PORTB on the XL/XE machines are:
Note: The 1200XL does not have BASIC built-in.
Memory management unit
A memory management unit , sometimes called paged memory management unit , is a computer hardware component responsible for handling accesses to memory requested by the CPU...
chip for the Atari 8-bit computers. It enables access to the hardware registers on ANTIC
ANTIC
Alphanumeric Television Interface Controller is an early video system chip used in the Atari 8-bit family of microcomputers as well as the Atari 5200 in the 1980s. The chip was patented by Atari, Inc. in 1981...
, GTIA, POKEY
Pokey
Pokey may refer to:* Pokey the Penguin, a surrealistic online comic strip* Pokey , a character from the Gumby television series...
and 6520 PIA.
The later XL/XE MMU (C061618) also selects OS
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
, Atari BASIC
Atari BASIC
Atari BASIC is a BASIC interpreter for the Atari 8-bit family of 6502-based home computers. The interpreter originally shipped on an 8 KB cartridge; on later XL/XE model computers it was built in, with an option to disable it, and started when the machines were booted with no other cartridges...
ROM, self-test ROM and LEDs in the 1200XL.
The user cannot directly manipulate the Atari MMU, but selects the various ROMS and memory via the memory-mapped hardware register known as PORTB (5401710 or D30116). Atari changed PORTB from an input port on the 400/800 machines to an output port on the XL/XE machines, leaving two joystick ports instead of four on the XL/XE machines.
By setting and clearing specific bits in PORTB, the user can access either the ROMs or memory locations. No synchronization is required as the OS handles the access.
The bit assignments for PORTB on the XL/XE machines are:
| PORTB (D30116) | |||
| Bit No. | Description | ||
| 0 | Controls OS ROM region. Disabling OS ROM enables RAM instead. 0 for disable, 1 for enable. | ||
| 1 | Controls built-in BASIC Atari BASIC Atari BASIC is a BASIC interpreter for the Atari 8-bit family of 6502-based home computers. The interpreter originally shipped on an 8 KB cartridge; on later XL/XE model computers it was built in, with an option to disable it, and started when the machines were booted with no other cartridges... . 0 for enable, 1 for disable. |
||
| 2 | Controls LED #1 in the 1200XL. Controls bank-selection in the 130XE. | ||
| 3 | Controls LED #2 in the 1200XL. Controls bank-selection in the 130XE. | ||
| 4 | CPU bank enable bit for 130XE only. | ||
| 5 | ANTIC bank enable bit for 130XE only. | ||
| 6 | Unused. | ||
| 7 | Controls the self test ROM region. Disabling self test enables RAM instead. 0 for enable, 1 for disable. | ||
Note: The 1200XL does not have BASIC built-in.
External links
- jindroush site(archived) MMU info
- INSIGHT: Atari - Compute! Magazine - Talks about Bank Selection in the Atari XL machines.