Multiple CPU language entry
With careful selection of opcodes a ROM's language entry can be made to execute both as 6502 code and in the ROM's destination code. Here are some examples of how to so this.
Z80
6502 code Z80 code 8000 18 CLC JR &8092 ; Z80 jumps to &8092 8001 90 BCC &8002+xx ; 6502 jumps to &8002+byte at &8002 8002 xx
6502 code Z80 code
8000 18 CLC JR &803A ; Z80 jumps to &803A
8001 38 SEC
8002 B0 BCS &8050 ; 6502 jumps to &8050
8003 4C ; 6502 entry overlaps service entry
; which must be &4C
32016
When entering 32016 code, the entry at &8000 is ignored, code is entered at the execution address stored after the Tube transfer address. If the ROM type indicates that there is no Tube transfer address, the execution offset is still used.
8000 .. .. .. ; 6502 enters at &8000
8003 .. .. .. ; 6502 service entry point
8006 EB cc vv ; Type, copyright offset, version
8009 nn nn nn ; ROM title
80cc 00 28 43 29 ... ; Copyright string
80xx 00
80yy rr rr rr rr ; Tube transfer address
80zz ee ee ee ee ; 32016 execution address offset
; Code is entered at rrrrrrrr + eeeeeeee
PDP11
As with the 32016, when entering PDP11 code, the entry at &8000 is ignored, code is entered at the execution address stored after the Tube transfer address. If the ROM type byte indicates that there is no Tube transfer address, then code is entered at &8000.
8000 .. .. .. ; 6502 enters at &8000
8003 .. .. .. ; 6502 service entry point
8006 E7 cc vv ; Type, copyright offset, version
8009 nn nn nn ; ROM title
80cc 00 28 43 29 ... ; Copyright string
80xx 00
80yy rr rr rr rr ; Tube transfer address
80zz ee ee ee ee ; PDP-11 execution address offset
; Code is entered at rrrrrrrr + eeeeeeee
ARM
See App note: ARM code in sideways ROMs (www.sprow.co.uk)
80186
80186 code 8000 E9 SBC #xx JMP &8002+xx 8001 xx
