EMMU
EMMU is a custom MMU chip for the 130XE Atari 8-bit computer. It is used to access the extra 64kB of RAM present in the 130XE.
The user cannot directly manipulate the EMMU, but selects the various ROMS and memory banks 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 the extra 64kB of memory in 16kB increments. No synchronization is required as the OS handles the access.
The bit assignments for PORTB are as follows:
| PORTB (D30116) | |||
| Bit No. | Description | ||
| 0 | Controls OS ROM region. Disabling OS ROM enables RAM instead. | ||
| 1 | Controls built-in BASIC. 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. | ||
See also
References
- Chadwick, Ian (1985). Mapping the Atari Revised Edition. COMPUTE! Publications, Inc. ISBN 0-87455-004-1.
External links
- jindroush site(archived) EMMU info, part no. C025953
- INSIGHT: Atari - Compute! Magazine - Talks about Bank Selection in the Atari XL machines.
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