What Register Does The Cmp Command Change
Assembly - Conditions
Conditional execution in associates linguistic communication is achieved by several looping and branching instructions. These instructions tin can change the menstruum of control in a program. Provisional execution is observed in two scenarios −
| Sr.No. | Conditional Instructions |
|---|---|
| 1 | Unconditional spring This is performed by the JMP pedagogy. Provisional execution often involves a transfer of control to the address of an educational activity that does non follow the currently executing didactics. Transfer of control may be frontwards, to execute a new fix of instructions or backward, to re-execute the same steps. |
| 2 | Conditional jump This is performed by a set of jump instructions j<condition> depending upon the condition. The conditional instructions transfer the command past breaking the sequential menstruum and they do information technology by changing the offset value in IP. |
Let united states talk over the CMP teaching earlier discussing the conditional instructions.
CMP Instruction
The CMP instruction compares two operands. It is generally used in provisional execution. This educational activity basically subtracts one operand from the other for comparing whether the operands are equal or not. Information technology does non disturb the destination or source operands. It is used along with the provisional jump instruction for conclusion making.
Syntax
CMP destination, source
CMP compares 2 numeric data fields. The destination operand could be either in register or in memory. The source operand could be a constant (firsthand) data, register or memory.
Example
CMP DX, 00 ; Compare the DX value with nothing JE L7 ; If yes, then spring to label L7 . . L7: ...
CMP is often used for comparing whether a counter value has reached the number of times a loop needs to be run. Consider the following typical condition −
INC EDX CMP EDX, ten ; Compares whether the counter has reached x JLE LP1 ; If it is less than or equal to x, then bound to LP1
Unconditional Jump
Equally mentioned before, this is performed by the JMP education. Provisional execution frequently involves a transfer of control to the address of an instruction that does not follow the currently executing instruction. Transfer of command may exist forward, to execute a new set up of instructions or backward, to re-execute the same steps.
Syntax
The JMP instruction provides a characterization name where the flow of control is transferred immediately. The syntax of the JMP instruction is −
JMP label
Instance
The post-obit code snippet illustrates the JMP instruction −
MOV AX, 00 ; Initializing AX to 0 MOV BX, 00 ; Initializing BX to 0 MOV CX, 01 ; Initializing CX to 1 L20: Add together AX, 01 ; Increment AX Add BX, AX ; Add AX to BX SHL CX, 1 ; shift left CX, this in turn doubles the CX value JMP L20 ; repeats the statements
Conditional Jump
If some specified condition is satisfied in conditional spring, the command flow is transferred to a target instruction. There are numerous conditional jump instructions depending upon the condition and data.
Following are the conditional leap instructions used on signed information used for arithmetics operations −
| Didactics | Description | Flags tested |
|---|---|---|
| JE/JZ | Spring Equal or Jump Nada | ZF |
| JNE/JNZ | Spring not Equal or Jump Non Null | ZF |
| JG/JNLE | Jump Greater or Bound Not Less/Equal | OF, SF, ZF |
| JGE/JNL | Jump Greater/Equal or Bound Not Less | OF, SF |
| JL/JNGE | Jump Less or Jump Not Greater/Equal | OF, SF |
| JLE/JNG | Jump Less/Equal or Bound Not Greater | OF, SF, ZF |
Following are the conditional jump instructions used on unsigned data used for logical operations −
| Instruction | Clarification | Flags tested |
|---|---|---|
| JE/JZ | Jump Equal or Bound Zero | ZF |
| JNE/JNZ | Leap not Equal or Jump Non Cipher | ZF |
| JA/JNBE | Jump Above or Jump Not Beneath/Equal | CF, ZF |
| JAE/JNB | Bound Above/Equal or Jump Not Beneath | CF |
| JB/JNAE | Bound Below or Jump Not To a higher place/Equal | CF |
| JBE/JNA | Jump Below/Equal or Jump Non Above | AF, CF |
The post-obit conditional jump instructions have special uses and check the value of flags −
| Instruction | Description | Flags tested |
|---|---|---|
| JXCZ | Jump if CX is Zero | none |
| JC | Jump If Acquit | CF |
| JNC | Jump If No Carry | CF |
| JO | Leap If Overflow | OF |
| JNO | Jump If No Overflow | OF |
| JP/JPE | Spring Parity or Jump Parity Even | PF |
| JNP/JPO | Jump No Parity or Jump Parity Odd | PF |
| JS | Jump Sign (negative value) | SF |
| JNS | Jump No Sign (positive value) | SF |
The syntax for the J<status> fix of instructions −
Case,
CMP AL, BL JE EQUAL CMP AL, BH JE EQUAL CMP AL, CL JE EQUAL NON_EQUAL: ... EQUAL: ...
Example
The post-obit program displays the largest of iii variables. The variables are double-digit variables. The three variables num1, num2 and num3 accept values 47, 22 and 31, respectively −
section .text global _start ;must be declared for using gcc _start: ;tell linker entry bespeak mov ecx, [num1] cmp ecx, [num2] jg check_third_num mov ecx, [num2] check_third_num: cmp ecx, [num3] jg _exit mov ecx, [num3] _exit: mov [largest], ecx mov ecx,msg mov edx, len mov ebx,i ;file descriptor (stdout) mov eax,4 ;system telephone call number (sys_write) int 0x80 ;call kernel mov ecx,largest mov edx, 2 mov ebx,1 ;file descriptor (stdout) mov eax,4 ;system call number (sys_write) int 0x80 ;call kernel mov eax, one int 80h section .data msg db "The largest digit is: ", 0xA,0xD len equ $- msg num1 dd '47' num2 dd '22' num3 dd '31' segment .bss largest resb 2
When the above code is compiled and executed, information technology produces the following result −
The largest digit is: 47
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Source: https://www.tutorialspoint.com/assembly_programming/assembly_conditions.htm
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