Generic Emulators

Barebone emulator, or “generic emulator”, is a customizable emulator of CPU + memory + serial terminal. You can configure it by simple config file. Let’s long story short…

First of all, prepare a config file, e.g. “mycomputer.emu”. Extension “.emu” is mandatory. File should contain this:

cpu Z80
memory.ram.from 0x1000
memory.ram.to 0x13ff
memory.rom.from 0x0000
memory.rom.to 0x0fff
;serial simple
;serial.in 1
;serial.out 1
;serial.status 0
;serial.status.available 0x20
;serial.status.ready 0x02
serial 6850
serial.data 0x81
serial.control 0x80
terminal.caps 1

Format is really simple: Each line contains one directive. Lines with “;” at the same beginning are comments. Here are parameters:

parameter

value (case sensitive)

cpu

can be “I8080”, “Z80” or “C6502”

memory.ram.from

starting address for RAM (please follow the 0x… convention for hexadecimal numbers)

memory.ram.to

last address of RAM

memory.rom.from, memory.rom.to

Same as above, but for ROM (your source code should lays here)

serial

Type of serial port. You can select “6850” or “simple”. See below

serial.data, serial.control

For 6850 serial – two ports.

serial.in, serial.out

in and out ports for “simple serial port”

serial.status

“Simple port” status. Read only port. It returns “serial.status.available” when there is any data for read, and “serial.status.ready” if port is ready to send a data.

terminal.caps

1 sets terminal to caps lock (default value 0 means “no caps”)

serial.interrupt

value 1 means that terminal invoke CPU interrupt on keypress (=serial data is ready for read). Default 0 means “no interrupt”.

Two serial ports are available – 6850 is the standard ACIA circuit, “simple” is a generic serial port with no complicated functions, just read and send bytes.

How to use it? It’s simple, just tell which emulator should use by the “.engine” directive (without “.emu”), like this:

org 0
.engine mycomputer
di
ld sp, 0x1400
ld a, 15h
out (80h),a
ld hl, hello
call printhl
endless:
call geta
jr z,endless
inc a
call printa
jr endless
printhl:
ld a,(hl)
or a
ret z
call printa
inc hl
jr printhl
printa:
push af
serdy:
in a,80h
and 02
jr z,serdy
pop af
out 81h,a
ret
geta:
in a,80h
and 01h
ret z
in a,81h
or a
ret
hello:
db "Hello World!",0x0d,0x0a,0

It’s a simple “hello” program, using the above configuration. Just save it as “test.z80” and click to “Emulate (F10)”. It should compile and run emulator with given configuration.

For 6502-based computer you can use this config file (e.g.”my6502.emu”):

cpu C6502
memory.ram.from 0x0000
memory.ram.to 0x0fff
memory.rom.from 0xf000
memory.rom.to 0xffff
serial 6850
serial.data 0xa001
serial.control 0xa000
serial.map 1

Please notice the “serial.map” directive. It means that serial port is not mapped into “I/O” space (like Z80/8080 does), but into memory space. Serial.data and serial.control are addresses now.

6502 requires RAM in a bottom part of address space, ROM at the top.

Try this code:

.engine my6502
ACIA = $A000
ACIACONTROL = ACIA+0
ACIASTATUS = ACIA+0
ACIADATA = ACIA+1
.ORG $FFFC
DW reset
DW reset
.org $f000
RESET:
LDX #$FF
TXS
LDA #$15
STA ACIAControl
LDY #0
LOOP:
LDA Message,Y
BEQ KEY
JSR SEROUT
INY
BNE LOOP
KEY: LDA ACIAStatus
AND #1
BEQ KEY
LDA ACIAData
; simple data mangling
SEC
ADC #0
JSR serout
JMP KEY
Message:
DB $0C,"My hovercraft is full of eels!",$0D,$0A,$00
SEROUT: PHA
SO_WAIT: LDA ACIAStatus
AND #2
BEQ SO_WAIT
PLA
STA ACIAData
RTS

Other processors and peripherals are on its way…