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Re: Help me with a crash theory?

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Hi,

>In both cases, reading the DATA register without a care for the
>STATUS register telling me a byte is ready just happens to work safely
>with numbers from $60 to $7F, but can wreak havoc with numbers from $64
>to $83.

Replying to myself: that theory is valid.

I tested it using a sender on the PC, doing
char buf[10000];

for (i = 0; i < 10000; i++)
  buf[i] = (buf % 2) ? 0xf0 : 0x0f

while(1)
  fwrite(buf, 1, 10000, tty);


On the Apple II, a basic cc65 program doing:
while (1) {
   char r = read_stuff();
   printf("unexpected %02x\n", r);
}

with read_stuff() being:

_read_stuff:
:       lda     $C0A9           ; Wait for first byte
        and     #$08
        beq     :-

        ; loop, reading data reg directly
duty_cycle:
        lda     $C0A8           ; 4 - get data byte
        cmp     #$F0            ; 6
        beq     ok_f0           ; 8/9
        cmp     #$0F            ; 10
        beq     ok_0f           ; 12/13

        ldx     #$00            ; We got something else. Return it.
        rts

ok_f0:  WASTE_63                ; 72 (9+63)
        bra     duty_cycle      ; 75

ok_0f:  WASTE_59                ; 72 (13+59)
        bra     duty_cycle      ; 75

Mystery solved :)
-- 
Colin
https://www.colino.net/

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Help me with a crash theory? Colin Leroy-Mira <colin@colino.net> - 2024-04-23 09:04 +0200
  Re: Help me with a crash theory? Colin Leroy-Mira <colin@colino.net> - 2024-04-23 17:54 +0200

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