Assembly cntd...

Typical assembly language instructions:
Made up of

Arithmetical instructions.

Logical instructions like 'AND', 'OR' and 'NOT'.

Rotate instructions.

Bit set or reset instructions.

Examples:

[add] [al,bl] - [;Add bl to al and store the result in al]
[sub] [ax, 08h] - [;Subtract 8 (hex) from ax and store the result in ax]
[mul] [bl] - [;al is multiplied by bl and stored in ax]
Note: ax is double the width of al and bl
[div] [cx] - [;The dx-ax register pair is divided by the cx with the quotient being placed in the ax register and the remainder being placed in the dx register]

Assembly Language

Second generation low-level language

Consists of four parts: [Label]-[Op Code]-[Operands]-[Comments]
eg [Start:]-[mov]-[bx,80h]-[;Load character count]

Label: Used as a reference and good for calculating relative jumps.

Operation Code: Contains mnemonic which describes operation.

Operands: Describe source and destination of the data to be operated on.

Comments: Used for convenience of programmer. Helps readability.

Advantages: Can be quicker than some high level languages.
- can access registers and exact memory addresses.

Addressing Modes:

Register addressing
- Data is transferred from a source register to a destination register
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Immediate addressing
- The data to be addressed is in the op code.
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Direct addressing
- Data is copied from a specified location in memory to the register.
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Indirect addressing
- Uses a number inside the register to point to location in memory.
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Indexed addressing
- Number within a register is used in combination with another number in another register to point to a location in memory.
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