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Memory Map
| Use | Start - End | Implementation |
|---|---|---|
| RAM | 0000 - DFFF | 56K NVRAM |
| D000 - D0FF | ASSIST09 working RAM, stacks and vector copies | |
| I/O | E000 - E3FF | Main Bus Select 0 1K |
| E400 - E7FF | Main Bus Select 1 1K | |
| E800 - EAFF | unassigned | |
| EB00 - EBFF | ACIA Select 256b | |
| EC00 - EEFF | unassigned | |
| EF00 - EFFF | Memory Latch Select 256b | |
| ROM | F000 - FFFF | EEPROM 4K |
| F000 - F3FF | ASSIST09 Expansion ROM area (unused) | |
| F800 - FFFF | ASSIST09 | |
| MPU | FFF0 - FFF1 | HD6390 Illegal Opcode/Divide By Zero Trap vector (FFD4) |
| FFF2 - FFF3 | SWI3 vector (FFD8) | |
| FFF4 - FFF5 | SWI2 vector (FFDC) | |
| FFF6 - FFF7 | /FIRQ vector (FFE0) | |
| FFF8 - FFF9 | /IRQ vector (FFE4) | |
| FFFA - FFFB | SWI vector (FFE8) | |
| FFFC - FFFD | /NMI vector (FFEC) | |
| FFFE - FFFF | /RESET vector (F837) |
The 2.0 board is hard wired for “Exxx” selects in the map above.
The subsequent version has jumpers to locate the I/O block
These use J3 select jumper for 8xxx - Exxx address select range
EEPROM 4K
The SST-6809 contains a 32KB EEPROM.
The Memory Latch Select allows mapping one of eight 4KB sections of the EEPROM into the memory range $F000-$FFFF which includes the MPU interrupt vectors.
Make use of the boot ROM select jumper(s).
The 2.0 can boot from 4 different segments, the 2.1 only 2.
But the current EEPROM only has ASSIST09 burned into it.
The “external ROM” is just the unused space in the other half of the 4k boot ROM.
Ken just never put anything there.
At some point Ken could burn another boot into an EEPROM.
The PTM on the Mezz board can support tracing with ASSIST09, but it needs a jumper to put its interrupt on NMI.
Return to SST-6809