Here I post more code. I tried to fix the horrible formatting a simple cut and paste winds up with. Hopefully I didn't create typos in that process.
EDIT: my formatting turned out no better. sorry
Here is the ISR. As you can see, it does as little as possible. I don't think it is related to the problem, as it will long have finished, and multiple JSR's called by the time the problem happens.
Code:
;***************************
;Interrupt service routine *
;***************************
isr: PHA
LDA COM1_STA ;get ACIA status register
BPL end_isr ;if no IRQ, return from ISR
LDA #$FF
STA READ_FLAG ;main loop pols this to see if a key has been presses
end_isr: PLA
RTI
Also, here is the snippet in my main loop that deals with the flag the ISR sets.
Code:
inf_loop: LDA READ_FLAG ;see if key was pressed. ISR sets this flag
BNE read_irq ;not 0? Flag was set
JMP inf_loop ;no flag. Loop as usual
read_irq: STZ READ_FLAG ;flag was set. Clear it
JSR terminal_r ;process the keypress
JMP inf_loop ;loop as usual
Finally, here is the terminal_r subroutine. This routine eventually calls process_cmdln, which is the troublesome routine. I edited my comments to reflect our conversation in a few spots, so look for that.
Code:
;**************************
;Terminal read subroutine *
;**************************
terminal_r:
PHA
PHX
PHY
LDX STRING_INDEX ;get string index. This is index is the current position in the input buffer
LDY COM1_DAT ;get data from ACIA Rx
CPY #$0D ;check if enter was pressed
BEQ process_cmd ;if enter pressed, process the input string
JSR com1_TX_chk ;wait til TX buffer clears
TYA ;I can't remember why I initially use Y, and then transfer it. Clue maybe?
STA COM1_DAT
STA COM1_RXBUF,X ;place recieved data into input buffer
INX ;increment it
STX STRING_INDEX ;put incremented version back in memory
PLY
PLX
PLA
RTS
process_cmd:
STZ COM1_RXBUF,X ;put NULL on end of string
JSR process_cmdln ;index arguments, if any. This is the subroutine that crashes
STZ STRING_INDEX ;reset index. When it crashes, this line is never reached
JSR parse_cmd ;parse the command
JMP (SELECTED_CMD) ;run routine for selected command
rtn_to_tr: LDA #<prompt ;get lower byte of prompt message
STA STRING_PTR ;store it in first byte of the string pointer
LDA #>prompt ;get upper byte of prompt message
STA STRING_PTR+1 ;store it in second byte of string pointer
JSR terminal_w ;display prompt
PLY
PLX
PLA
RTS
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or uninitialized/unpopulated memory. Often the errant PC will end up repeating a loop of some kind. But it is still fetching instructions and at least trying. 
