Secure coding

Instructions in x86 assembly

March 2, 2020 by Richard Azu

Introduction

This article will define three main categories of instructions in the x86 architecture. It will give examples in each of these categories and then show simulations on how some instructions are executed.

Instructions in x86

Instructions can be defined as the language used to command a computer architecture. The x86 instructions basically tell the processor what to do. They are generally categorized into:

• Arithmetic and logic instructions
• Control-flow instructions
• Data movement instructions

Arithmetic and logic instructions in x86

These instructions consist of arithmetic and logical operations. Arithmetic operation can be applied on numerical quantities to multiply, add, subtract or divide. The logic operations are basically the operations performed by logic gates and the main operations are AND, OR, XOR and NOT.

Arithmetic instructions

Basic instruction format

Operand1 is the destination operand and operand2 is the source operand. The destination operand can be either a memory location or a register. Operand2 is the source operand and can be a constant, a register or memory location.

Example 1:

• MOV AX, 77H ;Copy the hex value 77 to the accumulator
• ADD AX, 80H ; Add the constant hex value 80 to the value in the accumulator.

Operand 1: 0111 0111

Operand2: 1000 0000

Content of operand1 after ADD operation: 1111 01111

Note that there is no carry from the lower four bits (low nibble) to the higher four bits (high nibble). The Auxiliary or Adjust flag will not be set.

Figure 1: Contents of the accumulator (register AX) after ADD operation in example 1 is executed

Figure 2. Status of flags after AND operation in example 1 is executed

Operation to execute: ADD AX, 80H

AX = 77H

Operation = 77H + 80H

Result = F7H

AX = F7H = 0000 0000 1111 0111b

Flag ZF = 0 ; result is not zero

Flag CF = 0; There is no carry from AL to AH

Flag SF = 0; Result is not negative (signed)

Flag OF = 0; Both operands were 8 bits long. Results is also 8 bits long. There is no overflow.

Flag PF = 0; The number of set bits in the result is 7, which is odd. PF is set when the number of set bits is even.

Flag AF = 0; There is no carry from low nibble to high nibble.

Flag IF = 1; This means the processor is capable of handling interrupts. Otherwise the interrupts are ignored.

Flag DF = 0; The Direction flag controls the right-to-left or left-to-right direction of string processing. When set to 1, the string is processed from highest to lowest address, Otherwise, strings are processed beginning from lowest to highest address.

Logical instructions

Basic instruction format

AND operand1, operand2

OR   operand1, operand2

XOR operand1, operand2

NOT operand

For AND, OR and XOR instructions, operand1 is the destination operand and operand2 is the source operand. The destination operand can be either a memory location or a register. Operand2 is the source operand and can be a constant, a register or memory location. NOT instruction operands can either be a memory location or a register.

Example 2:

• MOV AX, 77H ;Copy the hex value 77 to the accumulator
• XOR AX, 80H

Operand 1: 0111 0111

Operand2: 1000 0000

Content of operand1 after XOR operation: 1111 0111

Table 1: Possible states for two inputs

Figure 3: The XOR logic has an OFF state when both inputs are OFF

Figure 4: The XOR logic has an ON state when only one of the inputs are ON

Figure 5: The XOR logic has an ON state when only one of the inputs are ON

Figure 6: The XOR logic has an OFF state when both inputs are ON

Table 2: Output of XOR logic with two inputs

Following from figures 3 to 6, it can be summarized that the XOR logic only returns true or 1 when either of the two inputs is ON. It returns 0 or shows an OFF state when both inputs are either 0 (OFF) or 1 (ON).

Control-flow instructions in x86

Control-flow instructions determine the next instruction to execute. The execution of control-flow operations does not allow sequential flow of the program. Control-flow instructions include conditional and unconditional jump instructions.

Jump instructions

Jumps are instructions that move control of the currently running program to a distant labelled instruction when a flag condition is met. Since status flags are sometimes modified by arithmetic instructions, jump instructions make use of them before execution.

Basic instruction format

JMP label

A label, either symbolic or numeric, refers to a memory location where an instruction is located. It is a convenient way of referring to an instruction in memory instead of referring to the memory address using an integer.

Example 3:

• JMP  START:
• MOV AX, 77H
• MOV AX, 30H
• START:
• MOV AX, 100H
• MOV BX, 100H

In example 3, registers AX and BX will never be initialized with the values 77H and 30H respectively. As soon as the program starts, control is transferred to the instruction on line 5. Registers AX and BX only get the values 100H each.

Figure 7: Content of register AX after the JMP instruction is executed

Figure 8: Content of register BX after the JMP instruction is executed

Data movement instructions in x86

Data movement instructions move data values from one location to another. The source and destination locations can be registers or memory. Data movement instructions include MOV, PUSH and POP among many others.

Basic instruction format

MOV operand1, operand2

Operand1 can either be a register or memory location. Operand2 can be a memory location, register or a constant value.

The MOV instruction moves the data item in the second operand into the first operand’s location. It is not possible to move data from one memory location to another memory location.

• PUSH operand1

Operand1 can be a memory location, register or a constant value

PUSH decrements the stack pointer by 2 if the operand-size attribute of the instruction is 16 bits; otherwise, it decrements the stack pointer by 4. PUSH then places the operand on the new top of stack, which is pointed to by the stack pointer.

• POP operand1

Operand1 can be a memory location or register

POP instructions perform the inverse operation of PUSH. It removes data from the stack and places it into the target register.

Conclusion

This article has explained the three main categories of instructions: arithmetic and logic instructions, control-flow instructions and data movement instructions. We’ve also shown simulations on how control-flow instructions and arithmetic and logic instructions are executed. These unique simulations show contents of specific registers and explains how status registers can be set during operations.

Sources

1. Instructions: Assembly Language, eceweb.ucsd.edu
2. PUSH — Push Operand onto the Stack, scs.stanford.edu
Posted: March 2, 2020
Richard Azu
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Experienced in the deployment of voice and data over the 3 media; radio, copper and fibre, Richard – a system support technician with First National Bank Ghana Limited is still looking for ways to derive benefit from the WDM technology in Optics. Using Kali as a springboard, he has developed an interest in digital forensics and penetration testing.