Pseudo-device “simh-pseudo”

This device is originally implemented as permanently connected to 88-SIO device in simh emulator. It listens to custom, non-standard commands from operating system thus simplifies communication between emulator (host) and running guest system. This device is required if you want to run CP/M operating system images made for simh emulator.

Programming

SIMH-pseudo device can be pretty useful also for users of emuStudio. Z80 or 8080 programs communicate with it via port 0xFE.

Some commands sent to the port require parameters, in which case there must be sent multiple bytes. If a command returns anything, the returned value can be read using IN instruction.

List of commands

In case of multibyte parameters or return value, if it’s a numeric value it’s always in a form of little endian.

How to call a command

Calling a command always starts with an OUT instruction:

ld  a, <cmd>  ; replace "<cmd>" with command number
out (0xFE), a

Then, if a command requires a parameter, it must be supplied with additional OUT instruction(s), depending on how many bytes are expected, as follows:

ld a, <param0>  ; replace "<param0>" with parameter byte 0
out (0xFE), a
ld a, <param1>  ; replace "<param1>" with parameter byte 1
out (0xFE), a
...              ; etc.

After last parameter is recognized by SIMH-pseudo device, the command is executed. Then, if the command returns a result, it must be read with IN instructions, depending on how many bytes are to be returned, as follows:

in a, (0xFE)    ; register A contains first byte of result
...             ; save the byte somewhere
in a, (0xFE)    ; register A contains second byte of result

Note: The program must send/receive all bytes. Otherwise, the device will stay in a state when it “expects” the rest of parameter bytes, or result bytes to be read. It is however possible to “reset” the device by sending a reset command to the device (command 14).