Spectrum ROM: Routine at 13579

13579: THE 'ZERO OR ONE' SUBROUTINE

Used by the routines at e_to_fp, f_not, no_or_no and no_and_no.

The routine at less_0 continues here.

This subroutine sets the 'last value' to zero if the carry flag is reset and to one if it is set. When called from e_to_fp however it creates the zero or one not on the stack but in mem-0.

Input
HL	Address of the first byte of the number
F	Carry set for 1, reset for 0

FP_0_1

13579

PUSH HL

Save the result pointer.

13580

LD A,0

Clear A without disturbing the carry.

13582

LD (HL),A

Set the first byte to zero.

13583

INC HL

Point to the second byte.

13584

LD (HL),A

Set the second byte to zero.

13585

INC HL

Point to the third byte.

13586

RLA

Rotate the carry into A, making A one if the carry was set, but zero if the carry was reset.

13587

LD (HL),A

Set the third byte to one or zero.

13588

RRA

Ensure that A is zero again.

13589

INC HL

Point to the fourth byte.

13590

LD (HL),A

Set the fourth byte to zero.

13591

INC HL

Point to the fifth byte.

13592

LD (HL),A

Set the fifth byte to zero.

13593

POP HL

Restore the result pointer.

13594

RET