Spectrum ROM: Routine at 1F54

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.
BREAK_KEY	1F54	LD A,$7F	Form the port address +7FFE and read in a byte.
	1F56	IN A,($FE)	Form the port address +7FFE and read in a byte.
	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)	Form the port address +FEFE and read in a byte.
	1F5E	RRA	Again examine bit 0.
	1F5F	RET	Return with carry reset if both keys are being pressed.