Register Flags, Jumps & Conditions

COMP124 Lectures 2021-02-17

The Flags Resgister

We often want to query the effect of a previous instruction. The status of an operation is recorded in the flags register.

It contains these flags and others:

• S - Sign
  • Indicates whether result is +/-.
• Z - Zero
  • Indicates if result is zero or not.
• C - Carry
  • Indicates an arithmetic carry.
• O - Overflow
  • Arithmetic overflow error.

We can use these flags in conjunction with jump instructions to control program flow.

Jump Instructions

The simplest form is the unconditional jump. Is has the syntax:

JMP <address of the target instruction>

The address of the target instruction is usually given as a label.

Example

start: ...
	...
	jmp continue	; jump forwards
	...
	...
	jmp start 	; jump backwards
	...
	...
continue:

Jumping is unrestricted, so programmers must take care to avoid spaghetti code.

Conditional Jumps

Conditional jumps make the jump only if a specified condition holds.

If the condition is false, execution “falls through” to the next instruction.

Jumps that Test Flags

Instruction	Jumps if	Value
JC	Carry flag is set	1
JNC	Carry flag is clear	0
JZ	Zero flag is set	1
JNZ	Zero flag is clear	0
JS	Sign flag is set	1
JNS	Sign flag is not set	0
JO	Overflow flag is set	1
JNO	Overflow flag is not set	0

Example

num = num - 10;
if (num == 0) {
	num = 100;
}

The prior code would look like this in assembly:

	mov eax, num
	sub eax, 10
	jnz store
	mov eax, 100
store:
	mov num, eax

You can see that we should test if not zero in the assembly code such that the body of the if is executed if the branch doesn’t take place.

Comparing Values

The cmp instruction is the most common way of comparing two values, prior to a conditional jump.

Internally, it subtract one value from the other, without altering the operands.

It does modify the status flags.

Hence if eax and ebx contain the same values, then:

cmp eax, ebx

will set the Z flag.

Example

Suppose our program reads a single letter command from the user into the al register.

The command q is used to quit the program.

graph TD
1[cmp al, 'q'] --> z["Z (flags register)"]
z --> j[jz end]

Other Conditional Jumps

These jumps can be used in combination with the cmp instruction.

Instruction	Jumps if (after using cmp)
JE	The first operand (in cmp) is equal to the second operand.
JNE	The first and second operands are not equal.
JG/JNLE	First operand is greater.
JLE/JNG	First operand is less or equal.
JL/JNGE	First operand is less.
JGE/JNL	First operand is greater or equal.

if Statements

if (c > 0)
	pos = pos + c;
else
	neg = neg + c;

This will convert to the following assembly:

	mov eax, c
	cmp eax, 0 	; compare c to 0
	jg positive	; jump to positive if positive
negative:	; else negative
	add neg, eax
	jmp endif ; jump over positive
positive:
	add pos, eax
endif:
	...