Difference between revisions of "Z80 Routines:Optimized:reverseA"

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(Commented the black magiks)
Line 35: Line 35:
 
;author: calcmaniac84
 
;author: calcmaniac84
 
ReverseA:
 
ReverseA:
ld b,a
+
ld b,a ;b=ABCDEFGH
rrca
+
rrca ;a=HABCDEFG
rrca
+
rrca ;a=GHABCDEF
xor b
+
xor b \ and %10101010 \ xor b ;a=GBADCFEH
and %10101010
+
ld b,a ;b=GBADCFEH
xor b
+
rrca ;a=HGBADCFE
ld b,a
+
rrca ;a=EHGBADCF
rrca
+
rrca ;a=FEHGBADC
rrca
+
rrca ;a=CFEHGBAD
rrca
+
xor b \ and %01100110 \ xor b ;a=GFEDCBAH
rrca
+
rrca ;a=HGFEDCBA</nowiki>
xor b
+
and %01100110
+
xor b
+
rrca
+
ret</nowiki>
+

Revision as of 22:11, 21 May 2011

This is a faster/smaller (and also obfuscated) replacement for the normal reverse a.

Typical routine:

;input: b
;output: a
;25 bytes, 96 clock cycles
typicalreversea:
	rr b
	rla
	rr b
	rla
	rr b
	rla
	rr b
	rla
	rr b
	rla
	rr b
	rla
	rr b
	rla
	rr b
	rla
	ret

Better routine:

;- Reverse a
;input:	byte in A
;output: Reversed byte in A
;destroys B
;18 bytes and 66 clock cycles
;author: calcmaniac84
ReverseA:
	ld b,a 		;b=ABCDEFGH
	rrca 		;a=HABCDEFG
	rrca 		;a=GHABCDEF
	xor b \ and %10101010 \ xor b 	;a=GBADCFEH
	ld b,a 		;b=GBADCFEH
	rrca 		;a=HGBADCFE
	rrca 		;a=EHGBADCF
	rrca 		;a=FEHGBADC
	rrca 		;a=CFEHGBAD
	xor b \ and %01100110 \ xor b 	;a=GFEDCBAH
	rrca 		;a=HGFEDCBA