Posted By
Mad
on 2017-04-23
18:44:48
|  Re: "Ram under Rom"
What Csabo said.
Referring to the basic code above:
The datas place this code to $0fe8 (4072)
.C:0fe8 78 SEI
.C:0fe9 8D 3F FF STA $FF3F
.C:0fec 20 00 80 JSR $8000
.C:0fef 8D 3E FF STA $FF3E
.C:0ff2 58 CLI
.C:0ff3 60 RTS
and this code to $8000
.C:8000 A9 32 LDA #$32
.C:8002 8D 19 FF STA $FF19
.C:8005 60 RTS
The first code is used to disable the interrupts (SEI) and turn on RAM (sta $ff3f)
then the routine jumps to the routine at $8000 and changes the bordercolor.
The wedge then takes care of reenabling the ROM(sta $ff3e) and turns on the interrupts again(CLI).
Perhaps you should install this wedge ($0fe8) somewhere in a unused zeropage area or in a place which is not touched by the system and then exchange only the $8000 in the JSR instruction for your needs on the fly. You can call the same wedge routine (which is on a safe place), for all your basic SYS calls which should go into the ram under the rom.
edit: oh my, surely the place $0fe8 choosen by csabo is already sufficient for persistence. It will not be overwritten by anything systemwise. just for the record :).. Nice place for a very small routine :)..
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