AMD x86 manual AMD Athlon Processor Specific Code, Blended AMD-K6and AMD Athlon Processor Code

AMD Athlon™ Processor x86 Code Optimization 22007E/0 — November 1999

AMD Athlon™ Processor Specific Code

Example 1 — Signed integer ABS function (X = labs(X)):

MOV	ECX, [X]	;load value
MOV	EBX, ECX	;save	value
NEG	ECX	;–value
CMOVS	ECX, EBX	;if –value		is negative, select value
MOV	[X], ECX	;save	labs	result

Example 2 — Unsigned integer min function (z = x < y ? x : y):

MOV	EAX, [X]		;load X		value
MOV	EBX, [Y]		;load	Y	value
CMP	EAX, EBX		;EBX<=EAX			? CF=0 : CF=1
CMOVNC	EAX,	EBX	;EAX=(EBX<=EAX) ? EBX:EAX
MOV	[Z],	EAX	;save	min		(X,Y)

Blended AMD-K6®and AMD Athlon™ Processor Code

Example 3 — Signed integer ABS function (X = labs(X)):

MOV	ECX, [X]	;load value
MOV	EBX, ECX	;save value
SAR	ECX, 31	;x < 0	? 0xffffffff : 0
XOR	EBX, ECX	;x < 0	? ~x : x
SUB	EBX, ECX	;x < 0	? (~x)+1 : x
MOV	[X], EBX	;x < 0	? -x : x

Example 4 — Unsigned integer min function (z = x < y ? x : y):

MOV	EAX, [x]	;load x
MOV	EBX, [y]	;load y
SUB	EAX, EBX	;x < y ? CF : NC ; x - y
SBB	ECX, ECX	;x < y ? 0xffffffff : 0
AND	ECX, EAX	;x < y ? x - y : 0
ADD	ECX, EBX	;x < y ? x - y + y : y
MOV	[z], ECX	;x < y ? x : y

Example 5 — Hexadecimal to ASCII conversion (y=x < 10 ? x + 0x30: x + 0x41):

MOV	AL, [X]		;load	X value
CMP	AL,	10	;if x	is	less	than 10, set carry flag
SBB	AL,	69h	;0..9	–>	96h,	Ah..Fh	–> A1h...A6h
DAS			;0..9: subtract 66h,				Ah..Fh: Sub.	60h
MOV	[Y],AL		;save	conversion in y

58 Avoid Branches Dependent on Random Data