Compilation Basics (4) Program Control transfer

Program Control transfer

For programming to control the direction is very important thing, it is your program according to the conditions

Make a judgment and jump to the corresponding bit value. Unconditional Jump

The most basic instructions for steering the control program are jjmp.

Use the following form:
JMP Label
The way to declare/label/in a program is simple, just add ":" after its name,
A label can be mixed by any character but cannot start with a number, for example, the following
It's a 3 legal label.
Label1:
Label2:
A:
A label can be declared before an instruction, for example:
X1:
MOV AX, 1

X2:mov AX, 2
Here is an example of a JMP directive:

ORG 100h

MOV AX, 5; Set AX to 5.
MOV BX, 2; Set BX to 2.

JMP Calc; Jump to ' calc '.

BACK:JMP stop; Jump to ' stop '.

Calc:
ADD AX, BX; Add BX to AX.
JMP back; Return ' back '.

Stop

RET; Back to operating system

END

Of course there is a simpler way of calculating the sum of these two numbers, but

The above is a good example of a JMP directive.

As you can see from the example, JMP can control both forward and backward. It can be

Move to any location (65535 bytes) of the current code snippet.

Short term transfer

Unlike the unconditional transfer directive in jmp, there are some conditional jumps

Directive (only if the condition of the directs jump). These directives are divided into

Three groups, the first group is only the detection of individual marker bits, the second set of comparative characters

Number, and the third group compares unsigned numbers.

Transfer instructions for detecting individual marker bits

Instructions	Description	Conditions	Opposite instruction
JZ, JE	If 0 (equal), transfer.	ZF =1	JNZ, JNE
JC, JB, Jnae	If carry (less than, not greater than or equal to), transfer	CF = 1	JNC, JNB, JAE
Js	If it is a negative number, transfer	SF = 1	JNS
JO	If overflow, transfer	of = 1	Jno
JPE, JP	If it is an even number, transfer	PF = 1	JPO
JNZ, JNE	If not 0 (not equal), transfer	ZF = 0	JZ, JE
JNC, JNB, JAE	If there is no carry (greater than, greater than or equal), the transfer	CF = 0	JC, JB, Jnae
JNS	If not negative, transfer	SF = 0	Js
Jno	If there is no overflow, transfer	of = 0	JO
JPO, JNP	If not even, transfer	PF = 0	JPE, JP

You can see that some instructions function the same, yes, they build after compiling

Same machine code so it's easy to understand why you compiled JE instructions

The anti-compilation is JZ. A different name is used to make the program easier to understand.

Comparing the transfer instructions with signed numbers

Instructions	Description	Conditions	Opposite instruction
JE, JZ	if equal to (=), if 0, jump	ZF = 1	JNE, JNZ
JNE, JNZ	If not equal to (<>), if not equal to 0, jump	ZF = 0	JE, JZ
JG, Jnle	If greater than (>) if not less than equals (not <=), jump	ZF = 0 And SF = of	JNG, Jle
JL, Jnge	If the small and jump if less (<) is not greater than or equal to (not >=), jumps	SF <> of	JNL, Jge
Jge, JNL	If greater than or equal (>=), if not less than (not <), jump	SF = of	Jnge, JL
Jle, JNG	If less than equals (<=), if not greater than (not >), jump	ZF = 1 Or SF <> of	Jnle, JG

<>-symbol indicates not equal.

Compare unsigned number transfer directives

Instructions	Description	Conditions	Opposite instruction
JE, JZ	if equal to (=)., if 0, jump	ZF = 1	JNE, JNZ
JNE, JNZ	If not equal to (<>), if not 0, jump	ZF = 0	JE, JZ
JA, Jnbe	If greater than (>), if not less than equals (not <=), jump	CF = 0 and ZF = 0	JNA, Jbe
JB, Jnae, JC	If less than (<), if not greater than equals (not >=), if rounding, jump	CF = 1	JNB, JAE, JNC
JAE, JNB, JNC	If greater than or equal (>=), if not less than (not <), if no rounding, jump	CF = 0	Jnae, JB
Jbe, JNA	If less than or equal to (<=), if not greater than (not >), jump	CF = 1 Or ZF = 1	Jnbe, JA

In general, you need to use CMP directives to compare numeric values (the directive and the SUB (subtraction)

The directive is similar, except that the result is not saved, but only the value bits are modified.

The above statement means, for example:

Need to compare 5 and 2, 5-2 =3

The result is not 0 (0 value bit set to 0)

Another example

Compare 7 and 7

7-7 = 0

The result is 0. (0 The standard value bit is set to 1.) JZ or JE will be transferred).

Here is an example of a CMP directive and conditional transfer directive: