Acorn 6809 Card Monitor API
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JSR [$0365] OSWRCH (COPADR) Console output               -> DISPLAY
JSR [$0367] OSRDCH (CINADR) Console input (from buffer)  -> GETCHR
JSR [$0369] OSBPUT (CASOPA) Cassette output              -> MCASOP
JSR [$036B] OSBGET (CASINA) Cassette input               -> MCASIN
JSR [$036D] OSPRNT (PRINTI) Printer output               -> PRINT
JSR $F871   OSCLI  (MEMUSE) Execute command line

OSWRCH: Send character to display
On entry: A=character to display
On exit:  all preserved

OSRDCH: Wait for character from keyboard
On entry: none
          ECHOF $0360 must be set to $00
On exit:  A=character from keyboard buffer

OSPRNT: Send character to printer
On entry: A=character to print
On exit:  all preserved

OSBPUT: Send byte to file
On entry: A=byte to write
On exit:  all preserved

OSBGET: Read byte from file
On entry: none
On exit:  A=byte read

OSCLI: Execute command line
On entry: X=>command line, ends with <00>
On exit:  X=corrupted
          A=$00 Ok, A<>$00 Error