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1F54: THE 'BREAK-KEY' SUBROUTINE
Used by the routines at COPY_LINE and STMT_RET.
This subroutine is called in several instances to read the BREAK key. The carry flag is returned reset only if the SHIFT and the BREAK keys are both being pressed.
Input
Output
BREAK_KEY 1F54 LD A,$7F Form the port address +7FFE and read in a byte.
1F56 IN A,($FE)
1F58 RRA Examine only bit 0 by shifting it into the carry position.
1F59 RET C Return if the BREAK key is not being pressed.
1F5A LD A,$FE Form the port address +FEFE and read in a byte.
1F5C IN A,($FE)
1F5E RRA Again examine bit 0.
1F5F RET Return with carry reset if both keys are being pressed.
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