notebook/notes/x86-64/instructions/conditions.md

title	TARGET DECK	FILE TAGS	tags
Condition Operations	Obsidian::STEM	x86-64	x86-64

Overview

A number of instructions operate with respect to the registers#Condition Codes.

The conventional way to implement conditional operationsis through a conditional transfer of control, where the program follows one execution path when a condition holds and another when it does not.

An alternate strategy is through a conditional transfer of data. This approach computes both outcomes of a conditional operation and then selects one based on whether or not the condition holds.

%%ANKI Basic What does the conditional transfer of control refer to? Back: Following one execution path when a condition holds and another when it does not. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What does the conditional transfer of data refer to? Back: Moving data between destinations based on a condition. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze With respect to assembly, we usually discuss either conditional transfer of {1:control} or {1:data}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What does "conditional transfer" refer to in the context of control? Back: The movement of the PC to a different address depending on conditions. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What does "conditional transfer" refer to in the context of data? Back: The actual moving of data between destinations if a condition is satisfied. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Why might conditional transfers of control be less performant than that of data? Back: Branch prediction penalties may be more expensive than computing both branches' instructions. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic When can we not use conditional transfer of data? Back: When a branch may yield an error or some other side effect. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic The following pseudocode is a demonstration of the conditional transfer of what?

v = then-expr;
ve = else-expr;
t = test-expr;
if (!t) v = ve;

Back: Data. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic The following pseudocode is a demonstration of the conditional transfer of what?

  if (!test-expr)
    goto false;
  v = then-expr;
  goto done;
false:
  v = else-expr;
done:

Back: Control. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Why couldn't we use conditional transfer of data with the following expression?

xp ? *xp : 0

Back: Dereferencing xp may throw a null pointer dereferencing error. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

END%%

Instructions

CMP and TEST

Instruction	Operands	Based On	Description
cmp[bwlq]	S_1, S_2	S_2 - S_1	Compare
test[bwlq]	S_1, S_2	S_1 \mathop{\&} S_2	Test

%%ANKI Basic What four variants do CMP instructions take on in x86-64? Back: cmpb, cmpw, cmpl, cmpq Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What instruction class is CMP "based" on? Back: SUB Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What registers does CMP affect? Back: The condition code registers. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What distinguishes CMP from SUB? Back: CMP does not update any destinations. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic How does Bryant et al. recommend reading CMP instructions? Back: As subtracting the first operand from the second. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What instruction class is TEST "based" on? Back: AND Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What distinguishes TEST from AND? Back: TEST does not update any destinations. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What four variants do TEST instructions take on in x86-64? Back: testb, testw, testl, testq Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {1:CMP} is to {2:SUB} as {2:TEST} is to {1:AND}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What registers does TEST affect? Back: The condition code registers. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

SET

The description of the SET commands apply to the case of a comparison instruction. That is, the condition codes are set according to computation t = a - b, where t, a, and b may be interpreted as signed or unsigned depending on the SET instruction invoked.

Instruction	Synonym	Evaluation	Description
sete	setz	ZF	Equal / zero
setne	setnz	~ZF	Not equal / not zero
sets		SF	Negative
setns		~SF	Nonnegative
setl	setnge	SF ^ OF	Less (signed <)
setle	setng	(SF ^ OF) | ZF	Less or equal (signed <=)
setg	setnle	~(SF ^ OF) & ~ZF	Greater (signed >)
setge	setnl	~(SF ^ OF)	Greater or equal (signed >=)
setb	setnae	CF	Below (unsigned <)
setbe	setna	CF | ZF	Below or equal (unsigned <=)
seta	setnbe	~CF & ~ZF	Above (unsigned >)
setae	setnb	~CF	Above or equal (unsigned >=)

Note how the other condition code evaluations are easy to derive from setl and setb.

%%ANKI Basic What arithmetic computation is a SET instruction's description based on? Back: t = a - b Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What value does a SET instruction assign to a destination? Back: 0 or 1 Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic How large is the destination of a SET instruction? Back: A single byte. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is q in the cmpq instruction short for? Back: Quad word. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is e in the sete instruction short for? Back: Equal. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is z in the setz instruction short for? Back: Zero. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {sete} is a synonym for {setz}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is ne in the setne instruction short for? Back: Not equal. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is nz in the setnz instruction short for? Back: Not zero. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {setne} is a synonym for {setnz}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is s in the sets instruction short for? Back: Signed. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is ns in the setns instruction short for? Back: Not signed. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {set[ez]} is to {ZF} whereas {sets} is to {SF}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does sete refer to? Back: ZF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does setz put in its specified destination? Back: ZF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does setnz refer to? Back: ZF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does setne put in its specified destination? Back: ~ZF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does sets refer to? Back: SF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does sets put in its specified destination? Back: SF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does setns refer to? Back: SF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does setns put in its specified destination? Back: ~SF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is l in the setl instruction short for? Back: Less. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {setl} is a synonym for {setnge}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is nge in the setnge instruction short for? Back: Not greater or equal. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is b in the setb instruction short for? Back: Below. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {setb} is a synonym for {setnae}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is nae in the setnae instruction short for? Back: Not above or equal. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {1:setl} is to {2:signed} integers whereas {2:setb} is to {1:unsigned} integers. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {1:setnae} is to {2:unsigned} integers whereas {2:setnge} is to {1:signed} integers. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does setl refer to? Back: SF and OF. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does setl put in its specified destination? Back: SF ^ OF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does setb refer to? Back: CF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does setb put in its specified destination? Back: CF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is le in the setle instruction short for? Back: Less or equal. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {setle} is a synonym for {setng}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {1:setle} is to {2:signed} integers whereas {2:setbe} is to {1:unsigned} integers. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {1:setna} is to {2:unsigned} integers whereas {2:setng} is to {1:signed} integers. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does setle refer to? Back: SF, OF, and ZF. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does setle put in its specified destination? Back: (SF ^ OF) | ZF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is g in the setg instruction short for? Back: Greater. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {setg} is a synonym for {setnle}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {1:setg} is to {2:signed} integers whereas {2:seta} is to {1:unsigned} integers. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {1:setnle} is to {2:signed} integers whereas {2:setnbe} is to {1:unsigned} integers. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does setg refer to? Back: SF, OF, and ZF. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does setg put in its specified destination? Back: ~(SF ^ OF) & ~ZF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is ge in the setge instruction short for? Back: Greater or equal. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {setge} is a synonym for {setnl}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {1:setge} is to {2:signed} integers whereas {2:setae} is to {1:unsigned} integers. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {1:setnb} is to {2:unsigned} integers whereas {2:setnl} is to {1:signed} integers. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does setge refer to? Back: SF and OF. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does setge put in its specified destination? Back: ~(SF ^ OF) Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is a in the seta instruction short for? Back: Above. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {seta} is a synonym for {setnbe}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does seta refer to? Back: CF and ZF. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does seta put in its specified destination? Back: ~CF & ~ZF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is ae in the setae instruction short for? Back: Above or equal. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {setae} is a synonym for {setnb}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does setae refer to? Back: CF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does setae put in its specified destination? Back: ~CF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is be in the setbe instruction short for? Back: Below or equal. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze {setbe} is a synonym for {setna}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What condition code(s) does setbe refer to? Back: CF and ZF. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In terms of condition codes, what value does setbe put in its specified destination? Back: CF | ZF Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

JMP

Jump instructions are categorized as either direct or indirect. Direct jump instructions specify a label whereas indirect jump instructions specify a * followed by an memory operand.

Instruction	Synonym	Jump Condition	Description
jmp Label	-	1	Direct jump
jmp *Operand	-	1	Indirect jump
je Label	jz	ZF	Equal / zero
jne Label	jnz	~ZF	Not equal / not zero
js Label	-	SF	Negative
jns Label	-	~SF	Nonnegative
jl Label	jnge	SF ^ OF	Less (signed <)
jle Label	jng	(SF ^ OF) | ZF	Less or equal (signed <=)
jg Label	jnle	~(SF ^ OF) & ~ZF	Greater (signed >)
jge Label	jnl	~(SF ^ OF)	Greater or equal (signed >=)
jb Label	jnae	CF	Below (unsigned <)
jbe Label	jna	CF | ZF	Below or equal (unsigned <=)
ja Label	jnbe	~CF & ~ZF	Above (unsigned >)
jae Label	jnb	~CF	Above or equal (unsigned >=)

%%ANKI Basic How is the operand of a direct jump formatted? Back: As a label. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic How is the operand of an indirect jump formatted? Back: As an * followed by a register or memory address. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Is the following considered a direct or indirect jump instruction?

jmp *(%rax)

Back: Indirect. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Is the following considered a direct or indirect jump instruction?

jmp .L1

Back: Direct. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Is the following considered a direct or indirect jump instruction?

jmp *%rax

Back: Indirect. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Is the following considered a direct or indirect jump instruction?

jg *%rax

Back: N/A. This is not a valid instruction. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is a jump target? Back: An address of some destination instruction specified by a jump instruction. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What restriction is enforced on the operand of nonconditional jump instructions? Back: N/A. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What restriction is enforced on the operand of conditional jump instructions? Back: Conditional jumps can only be direct. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic A PC-relative jump encoding encodes the difference between what two addresses? Back: The jump target and that of the instruction immediately following the jump. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is "PC" in "PC-relative" short for? Back: Program Counter. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic How large is a PC-relative jump offset encoding? Back: 1, 2, or 4 bytes. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze A {PC-relative} jump encoding contrasts an {absolute} jump encoding. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic How large is an absolute jump address encoding? Back: 4 bytes. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Why is PC-relative addressing performed relative to the instruction after a jump? Back: It is convention to update the PC as the first step of executing any instruction. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Why is PC-relative encoding more compact than absolute encoding? Back: Offsets may be expressable in 1 or 2 bytes instead of 4. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Why is PC-relative encoding more portable than absolute encoding? Back: Because object code can shift position in memory without alteration. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is the nonconditional direct jump instruction? Back: jmp[bwlq] Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is the nonconditional indirect jump instruction? Back: jmp[bwlq] Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Conditional transfer of control usually involves what instructon set? Back: JMP Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic In most cases, how can the following for loop be rewritten as a while loop?

for (init; test; update) { body; }

Back:

init;
while (test) { body; update }

Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

END%%

%%ANKI Basic In what situation are the following two blocks of code not equivalent?

for (init; test; update) { body; }

init;
while (test) { body; update }

Back: Situations in which the for-loop has a continue statement in the body. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

END%%

%%ANKI Basic What efficient data structure do switch statements compile down into? Back: Jump tables. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

END%%

%%ANKI Basic What two conditions must be satisfied before a switch statement employs a jump table? Back: A sufficient number of case statements that span a small range of values. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

END%%

%%ANKI Basic Why might a switch statements be faster than an equivalent if/else chain? Back: A switch statement can compile down into a jump table. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

END%%

%%ANKI Basic What is a jump table? Back: An array of (code) memory locations to potentially jump to. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Suppose a switch compiles into a jump table. What is a missing case? Back: An offset in the jump table corresponding to an unspecified case statement. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

END%%

%%ANKI Basic Suppose a switch compiles into a jump table. How are missing cases handled? Back: By using the label (memory location) for the default case. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

END%%

%%ANKI Basic Suppose a switch compiles into a jump table. How are duplicate cases handled? Back: By using the same label (memory location) for all duplicate cases. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016. Tags: c17

END%%

CMOV

Like access#MOV instructions, but with the data transfer only happening if the move condition is satisfied.

Instruction	Synonym	Move Condition	Description
cmove	cmovz	ZF	Equal / zero
cmovene	cmovnz	~ZF	Not equal / not zero
cmovs	-	SF	Negative
cmovns	-	~SF	Nonnegative
cmovl	cmovnge	SF ^ OF	Less (signed <)
cmovle	cmovng	(SF ^ OF) | ZF	Less or equal (signed <=)
cmovg	cmovenle	~(SF ^ OF) & ~ZF	Greater (signed >)
cmovge	cmovnl	~(SF ^ OF)	Greater or equal (signed >=)
cmovb	cmovnae	CF	Below (unsigned <)
cmovbe	cmovna	CF | ZF	Below or equal (unsigned <=)
cmova	cmovnbe	~CF & ~ZF	Above (unsigned >)
cmovae	cmovnb	~CF	Above or equal (Unsigned >=)

%%ANKI Cloze {1:MOV} is to {2:unconditional} whereas {2:CMOV} is to {1:conditional}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic Conditional transfer of data usually involves what instructon set? Back: CMOV Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Basic What is instruction cmove an acronym for? Back: Conditional move equal. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

%%ANKI Cloze Conditional transfer of {1:control} is to {2:JMP} and {2:data} is to {1:CMOV}. Reference: Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.

END%%

Bibliography

• Bryant, Randal E., and David O'Hallaron. Computer Systems: A Programmer's Perspective. Third edition, Global edition. Always Learning. Pearson, 2016.