Memory
The MSI/88e has an 8K system ROM and battery-backed CMOS RAM. Two RAM board variants were produced (10K/12K and 24K); see the PCB section. The firmware accesses the RAM in a window at 0x8000.
The exact ROM/RAM decode needs to be confirmed from the schematic - a footer note in the disassembler configuration (mem=2000, rom=F800) hints at a possible banked/high-end ROM mapping.
Memory Map
0000-1FFF |
System ROM (8 KB) - SCM23C65 / CM6400-3 2764-compatible CMOS ROM. Reset vector 0x0000 (DIS/IDL/OUT 1/OUT 2/OUT 4/IDL), initialisation continues at 0x1F60. |
8000-8FFF |
RAM - window used by the firmware (stack, buffers, tables). The exact size and decode depend on the RAM board variant and are to be confirmed from the schematic. |
RAM Usage (from the disassembly)
0x809C |
Stack (register R2 is initialised to this address at boot). |
0x8100 |
Scratch buffer (registers R6/RE). |
0x8103/0x8104 |
Program-loaded sync bytes 5A 5B (see the Program Loading section below). |
0x810B |
Display buffer of the self-test / idle program (receives the ERR string at boot). |
0x8203/0x8204 |
Entry vector of the loaded program. |
0x8205 |
Low dispatch-table page byte of the loaded program. |
0x8206/0x8209 |
Tables (registers R5/RA). |
Program Loading
The sync bytes 5A 5B at RAM 0x8103/0x8104 mark “a program is loaded”. Both the boot signature check (0x1FC3) and the interpreter restart entry (0x08BC) test this flag: when it is present, the loaded program is started through its entry vector at 0x8203/0x8204; when it is absent, the firmware runs the self-test / idle-display program at 0x1E2B and then deliberately powers down (DIS/IDL at 0x03DA) to save the batteries.
The ROM never writes the sync flag itself. An external loader - the host computer or a cassette download - stores the flag and the entry vector into the battery-backed RAM, which is how user programs survive power-off.
RAM Boards
| Board | Chips | Size |
PWB 27319-00-00 (10K/12K) |
5× Toshiba TC5517AP-8 2K×8 SRAM | 10 KB, one empty socket for the 12 KB option |
PWB 27274 (24K) |
12× Fujitsu MB8416A-15L 2K×8 SRAM | 24 KB (upgraded memory option) |
Both boards are battery-backed (lithium “keepers” cells switch the SRAM supply when the unit is off). The full chip inventory of the RAM boards is documented in the PCB section.
ROM Files
MSI88_2.BIN |
8,190 bytes | System ROM (SCM23C65 / HP CM6400-3), dumped 0x0000-0x1FFF. |