[Coco] 64K Coco mode and the MC6883

[Sean Conner]

It was thus said that the Great RETRO Innovations via Coco once stated:
> 
> Sean's note piques my interest, so I'll see if I can fire up the logic 
> analyzer (it'd be nice to have someone write a test program in ML that 
> checks all the test cases one right after the other, turning off IRQs, 
> and I can capture the entire address and data bus at the CPU and the SAM 
> or something for those cycles).  I would think it should also be easy to 
> test by writing a small program that does:
> 
> switch to all RAM mode and write 00 to the first byte of the various ROM 
> areas.
> 
> switch back to RAM/ROM mode and write a 01 to those same areas
> 
> switch back to all RAM mode and look at the various locations.  00 means 
> nothing got written, 01 means it did.

  Fair enough:  here's a BASIC program that should do just that:

10 CLEAR 999:FOR I=1 TO 59:READ B:P$=P$+CHR$(B):NEXT I:P=VARPTR(P$)+1:POKE P,126
20 PRINT"HIT ANY KEY TO RUN TEST";EXEC P;PRINT
30 A=253:M$="ROM-1":GOSUB100
31 A=A+1:M$="ROM-2":GOSUB100
33 A=A+1:M$='ROM-3":GOSUB100
40 END
90 DATA 52,3,173,159,160,0,39
91 DATA 250,26,80,79,183,255,223
92 DATA 183,128,0,183,160,0,183
93 DATA 192,0,76,183,255,222,183
94 DATA 128,0,183,160,0,183,192
95 DATA 0,183,255,223,182,128,0
96 DATA 151,253,182,160,0,151
97 DATA 254,182,192,0,151,255
98 DATA 183,255,222,53,131
100 PRINT M$;": ";
110 IF PEEK(A)=1 THEN PRINT "NO SWITCH NEEDED" ELSE PRINT "SWITCH NEEDED"
120 RETURN

  And here's the assembly code (the listing file from a custom 6809
assembler I wrote):

                         | FILE qtest.a
0000: 34    03         1 |         pshs    a,cc            
0002: AD    9FA000     2 | loop    jsr     [$A000] ; key press?
0006: 27    FA         3 |         beq     loop    ; wait until so
0008: 1A    50         4 |         orcc    #$50    ; disable IRQ
000A: 4F               5 |         clra            ; A = 0
000B: B7    FFDF       6 |         sta     $FFDF   ; set RAM/RAM
000E: B7    8000       7 |         sta     $8000   ; write to ROM-1
0011: B7    A000       8 |         sta     $A000   ; write to ROM-2
0014: B7    C000       9 |         sta     $C000   ; write to ROM-3
0017: 4C              10 |         inca            ; A = 1
0018: B7    FFDE      11 |         sta     $FFDE   ; set RAM/ROM
001B: B7    8000      12 |         sta     $8000   ; write to ROM-1
001E: B7    A000      13 |         sta     $A000   ; write to ROM-2
0021: B7    C000      14 |         sta     $C000   ; write to ROM-3
0024: B7    FFDF      15 |         sta     $FFDF   ; set RAM/RAM
0027: B6    8000      16 |         lda     $8000   ; read ROM-1
002A: 97    FD        17 |         sta     $FD     ; save
002C: B6    A000      18 |         lda     $A000   ; read ROM-2
002F: 97    FE        19 |         sta     $FE     ; save
0031: B6    C000      20 |         lda     $C000   ; read ROM-3
0034: 97    FF        21 |         sta     $FF     ;save
0036: B7    FFDE      22 |         sta     $FFDE   ; set RAM/ROM
0039: 35    83        23 |         puls    a,cc,pc ; return 

    2 | address     0002 loop

  Addresses $FD, $FE and $FF are unused by BASIC, so I used that to store
the results.  

  -spc