Here is the Tandy CoCo "ROM to RAM" program that was published back in the 1980s.
It copies the contents of the ROMs to the bank of RAM that can replace it.
Executing the address it's POKEd to will start running BASIC out of RAM.
I hand disassembled it, and commented the disassembly by looking up the opcodes in a book. I tried using the debugger in VCC, but it doesn't have a disassembler, and my internet was down so I couldn't download something else.
The BASIC and disassembly are included below. The BASIC program should should still work if you leave out lines 50, 60, and 70, but it will run a little slower.
BASIC:
REM MOVE ROM TO RAM ON A COCO 1 & 2
10 READ A$:IF A$="X" THEN END
20 POKE 20000+N,VAL("&H"+A$)
30 N=N+1:GOTO 10
40 DATA 34,01,1A,50,10,8E,80,00
REM LOOP TO COPY 6 BYTES AT A TIME. FASTER BUT NOT REQUIRED TO WORK
50 DATA B7,FF,DE,EC,A4,AE,22,EE
60 DATA 24,B7,FF,DF,ED,A1,AF,A1
70 DATA EF,A1,10,8C,F3,FC,25,E8
REM LOOP TO COPY 2 BYTES AT A TIME.
80 DATA 10,8C,FF,00,24,0C,B7,FF
90 DATA DE,EC,A4,B7,FF,DF,ED,A1
100 DATA 20,EE,35,01,39
110 DATA X
Assembly:
;*************************************************
;* Tandy CoCo 1 & 2 ROM to RAM
;* copies the contents of ROM to RAM and continues running it from there
;*************************************************
; start at location 20000 (code is position independant though)
org $4E20
; start of code
34,01 pshs $01 ; save cc register
1A,50 orcc $50 ; disable interrupts
10,8E,80,00 ldy $8000 ; point to ROM start address
loop1: ; copy 6 bytes at a time
B7,FF,DE sta $FFDE ; set ROM mode
EC,A4 ldd ,y ; read 6 bytes of ROM data
AE,22 ldx 2,y
EE,24 ldu 4,y
B7,FF,DF sta #$FFDF ; set RAM mode
ED,A1 std ,y++ ; write 6 bytes and update mem pointer
AF,A1 stx ,y++
EF,A1 stu ,y++
10,8C,F3,FC cmpy $F3FC ; are we done with 6 byte copy?
25,E8 blo loop1 ; go again if not
loop2: ; copy 2 bytes at a time
10,8C,FF,00 cmpy $FF00 ; are we done with 2 byte copy?
24,0C bhs exit ; branch if yes
B7,FF,DE sta $FFDE ; set ROM mode
EC,A4 ldd ,y ; read 2 bytes
B7,FF,DF sta $FFDF ; set RAM mode
ED,A1 std ,y++ ; write 2 bytes and update mem pointer
20,EE bra loop2 ; loop
exit:
35,01 puls $01 ; restore cc register
39 rts
