Spectrum ROM: Routine at 168F

168F: THE 'COLLECT A LINE NUMBER' SUBROUTINE

On entry the HL register pair points to the location under consideration. If the location holds a value that constitutes a suitable high byte for a line number then the line number is returned in DE. However if this is not so then the location addressed by DE is tried instead; and should this also be unsuccessful line number zero is returned.

Input
HL	Address of the first byte of the BASIC line number to test
DE	Address of the first byte of the previous BASIC line number

Output
DE	The line number, or zero if none was found

LINE_ZERO

168F

DEFB $00,$00

Line number zero.

LINE_NO_A

1691

EX DE,HL

Consider the other pointer.

1692

LD DE,$168F

Use line number zero.

The main entry point is here, and is used by the routines at ED_EDIT, ED_UP and LN_FETCH.

LINE_NO

1695

LD A,(HL)

Fetch the high byte and test it.

1696

AND $C0

1698

JR NZ,LINE_NO_A

Jump if not suitable.

169A

LD D,(HL)

Fetch the high byte and low byte and return.

169B

INC HL

169C

LD E,(HL)

169D

RET