toolchain/src/libc/swab.src at f814c9d22471bd661f862c7c74abbbd4aed43c5d · CE-Programming/toolchain · GitHub

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	.assume	adl=1

	.section	.text

	.global	_swab
	.type	_swab, @function

.if 1

; void swab(const void *__restrict src, void *__restrict dst, ssize_t count)
_swab:
	; swab has unspecified behaviour when count is odd:
	; - this implementation treats an odd count as if it were count - 1
	; - does nothing when count <= 1 or count is negative
	ld	iy, 0
	add	iy, sp
	sra	(iy + 11)
	ret	m		; do nothing when negative
	ld	bc, (iy + 9)
	rr	b
	rr	c
	; BC = count / 2
	sbc	hl, hl
	adc	hl, bc
	ret	z		; count <= 1
	ld	de, (iy + 6)	; DE = dst
	ld	hl, (iy + 3)	; HL = src
.L.loop:
	ld	a, (hl)		; A = src.lo
	inc	hl
	ldi			; dst.lo = src.hi
	ld	(de), a		; dst.hi = A
	inc	de
	jp	pe, .L.loop
	ret

.else

; void swab(const void *__restrict src, void *__restrict dst, ssize_t count)
_swab:
	; swab has unspecified behaviour when count is odd:
	; - this implementation will copy dst[count - 1] to src[count - 1] when count is odd
	; - does nothing when count <= 0 or count is negative
	ld	iy, 0
	add	iy, sp
	sra	(iy + 11)
	ret	m		; do nothing when negative
	ld	bc, (iy + 9)
	rr	b
	rr	c
	; BC = count / 2
	sbc	hl, hl
	adc	hl, bc
	; Carry is set when count is odd
	ld	de, (iy + 6)	; DE = dst
	ld	hl, (iy + 3)	; HL = src
	jr	z, .L.finish
.L.loop:
	ld	a, (hl)		; A = src.lo
	inc	hl
	ldi			; dst.lo = src.hi
	ld	(de), a		; dst.hi = A
	inc	de
	jp	pe, .L.loop
.L.finish:
	ret	nc		; even count
	; copy one more byte
	ld	a, (hl)
	ld	(de), a
	ret

.endif