Data Transfer Instructions in Computer Architecture: Mnemonic Tables, Types and Example

"Data transfer instructions in computer architecture" is a short-answer topic from the Central Processing Unit chapter of TU BSc CSIT Computer Architecture (CSC213, unit 5), where instructions are grouped into data transfer, data manipulation and program control. BIT151 Microprocessor and Computer Architecture covers the same grouping in unit 2, Instruction Set and Addressing Modes.

This article explains data transfer in computer architecture: what the instructions do, the typical mnemonics with their meanings, how data manipulation instructions differ, and a short worked example of how MOV, LD and ST behave differently. It closes with how the topic is answered for 5 marks. The unit notes are on the Central Processing Unit notes page.

Instruction types in a CPU

Most computers provide a large repertoire of instructions, but they fall into three groups.

Group	Purpose	Examples
Data transfer	Move data between registers, memory and I/O without changing the value	LD, ST, MOV, XCH, IN, OUT, PUSH, POP
Data manipulation	Perform arithmetic, logic and shift operations on data	ADD, SUB, AND, OR, XOR, SHR, SHL
Program control	Change the sequence in which instructions execute	JMP, BZ, CALL, RET, CMP, SKP

The first two groups work on data; the third works on the program counter.

Data transfer instructions

Data transfer instructions move data from one place to another and leave the value unchanged at the source. The locations involved are processor registers, memory words and the registers of I/O devices. Since the same instruction can reach its operand in different ways, a data transfer instruction is usually combined with an addressing mode such as immediate, direct or register indirect.

The typical set is the following. Mnemonics vary between machines, so state in the exam that these are typical.

Name	Mnemonic	Meaning
Load	LD	Transfer data from a memory location to a register, usually the accumulator
Store	ST	Transfer data from a register to a memory location
Move	MOV	Transfer data between registers, or between a register and memory
Exchange	XCH	Swap the contents of two registers, or a register and memory
Input	IN	Transfer data from an input port to a register
Output	OUT	Transfer data from a register to an output port
Push	PUSH	Place data from a register on the top of the stack
Pop	POP	Remove the top of the stack into a register

Load and store are the memory-facing pair: load reads memory into the processor, and store writes the processor's value back to memory. Move is the register-to-register copy. Input and output are the I/O transfers, and in a memory-mapped I/O system they are replaced by ordinary load and store to the device address.

Push and pop belong to the stack organisation. In a stack that grows toward lower addresses, PUSH decrements the stack pointer and then writes the value, and POP reads the value and then increments the stack pointer.

Data manipulation instructions

Data manipulation instructions change the value of the data, and they provide the computational power of the machine. They are divided into three groups.

Group	Typical mnemonics	Meaning
Arithmetic	INC, DEC, ADD, SUB, MUL, DIV, ADDC, SUBB, NEG	Increment, decrement, add, subtract, multiply, divide, add with carry, subtract with borrow, negate
Logical and bit manipulation	CLR, COM, AND, OR, XOR, CLRC, SETC, COMC	Clear, complement, bit-wise AND, OR, XOR; clear, set or complement the carry
Shift	SHR, SHL, SHRA, SHLA, ROR, ROL, RORC, ROLC	Logical shift, arithmetic shift, rotate, rotate through carry

The distinction to state in an answer is that data transfer instructions only copy the value, while data manipulation instructions compute a new value (and usually set status flags such as carry, zero and sign).

Worked example: MOV, LD and ST

Suppose memory location 200 holds 25, and register R1 holds 7. Consider three instructions in a one-address accumulator machine, with AC the accumulator.

The load instruction copies from memory into the accumulator. Executing it with address 200 gives:

$LD\; 200:\; AC \leftarrow M[200] = 25$

The memory word is unchanged, and the old content of AC is lost. The store instruction goes the other way. If AC now holds 25 and the instruction names address 300, then

$ST\; 300:\; M[300] \leftarrow AC = 25$

writes 25 into location 300, and AC keeps its value. The move instruction copies between registers without touching memory:

$MOV\; R2, R1:\; R2 \leftarrow R1 = 7$

Therefore R2 now holds 7 and R1 is still 7. The difference is where the data comes from and goes to: LD and ST each make one memory access, so they take longer than MOV, which stays inside the CPU and needs no memory cycle. In all three cases the value is only copied, not altered.

Data transfer in computer architecture: where it happens

Every data transfer takes place over some path. Inside the CPU it uses the internal buses or a common bus between registers; between the CPU and memory it uses the address, data and control buses; and between the CPU and peripherals it goes through I/O interfaces. The instruction types above are what the programmer sees, while the register transfer language $R2 \leftarrow R1$ is what the hardware does for each of them.

How to write it in the exam

For 5 marks, define data transfer instructions in one sentence (they move data between registers, memory and I/O without changing it), give the table of mnemonics with meanings, and add one line each on load, store and move. If the question is "data transfer and manipulation instructions", add the three groups of data manipulation with their mnemonics, and the sentence that separates the two kinds. A small LD, ST, MOV example earns the last marks.

Addressing modes decide how the operand of these instructions is found; they are covered in Instruction Set and Addressing Modes for BIT students. CSIT students can practise past questions on the CSC213 Computer Architecture page and relate transfers to the Register Transfer and Microoperations unit.

"Data transfer instructions in computer architecture" is a short-answer topic from the Central Processing Unit chapter of TU BSc CSIT Computer Architecture (CSC213, unit 5), where instructions are grouped into data transfer, data manipulation and program control. BIT151 Microprocessor and Computer Architecture covers the same grouping in unit 2, Instruction Set and Addressing Modes.

This article explains data transfer in computer architecture: what the instructions do, the typical mnemonics with their meanings, how data manipulation instructions differ, and a short worked example of how MOV, LD and ST behave differently. It closes with how the topic is answered for 5 marks. The unit notes are on the Central Processing Unit notes page.

Instruction types in a CPU

Most computers provide a large repertoire of instructions, but they fall into three groups.

Group	Purpose	Examples
Data transfer	Move data between registers, memory and I/O without changing the value	LD, ST, MOV, XCH, IN, OUT, PUSH, POP
Data manipulation	Perform arithmetic, logic and shift operations on data	ADD, SUB, AND, OR, XOR, SHR, SHL
Program control	Change the sequence in which instructions execute	JMP, BZ, CALL, RET, CMP, SKP

The first two groups work on data; the third works on the program counter.

Data transfer instructions

Data transfer instructions move data from one place to another and leave the value unchanged at the source. The locations involved are processor registers, memory words and the registers of I/O devices. Since the same instruction can reach its operand in different ways, a data transfer instruction is usually combined with an addressing mode such as immediate, direct or register indirect.

The typical set is the following. Mnemonics vary between machines, so state in the exam that these are typical.

Name	Mnemonic	Meaning
Load	LD	Transfer data from a memory location to a register, usually the accumulator
Store	ST	Transfer data from a register to a memory location
Move	MOV	Transfer data between registers, or between a register and memory
Exchange	XCH	Swap the contents of two registers, or a register and memory
Input	IN	Transfer data from an input port to a register
Output	OUT	Transfer data from a register to an output port
Push	PUSH	Place data from a register on the top of the stack
Pop	POP	Remove the top of the stack into a register

Load and store are the memory-facing pair: load reads memory into the processor, and store writes the processor's value back to memory. Move is the register-to-register copy. Input and output are the I/O transfers, and in a memory-mapped I/O system they are replaced by ordinary load and store to the device address.

Push and pop belong to the stack organisation. In a stack that grows toward lower addresses, PUSH decrements the stack pointer and then writes the value, and POP reads the value and then increments the stack pointer.

Data manipulation instructions

Data manipulation instructions change the value of the data, and they provide the computational power of the machine. They are divided into three groups.

Group	Typical mnemonics	Meaning
Arithmetic	INC, DEC, ADD, SUB, MUL, DIV, ADDC, SUBB, NEG	Increment, decrement, add, subtract, multiply, divide, add with carry, subtract with borrow, negate
Logical and bit manipulation	CLR, COM, AND, OR, XOR, CLRC, SETC, COMC	Clear, complement, bit-wise AND, OR, XOR; clear, set or complement the carry
Shift	SHR, SHL, SHRA, SHLA, ROR, ROL, RORC, ROLC	Logical shift, arithmetic shift, rotate, rotate through carry

The distinction to state in an answer is that data transfer instructions only copy the value, while data manipulation instructions compute a new value (and usually set status flags such as carry, zero and sign).

Worked example: MOV, LD and ST

Suppose memory location 200 holds 25, and register R1 holds 7. Consider three instructions in a one-address accumulator machine, with AC the accumulator.

The load instruction copies from memory into the accumulator. Executing it with address 200 gives:

$$LD; 200:; AC \leftarrow M[200] = 25$$

The memory word is unchanged, and the old content of AC is lost. The store instruction goes the other way. If AC now holds 25 and the instruction names address 300, then

$$ST; 300:; M[300] \leftarrow AC = 25$$

writes 25 into location 300, and AC keeps its value. The move instruction copies between registers without touching memory:

$$MOV; R2, R1:; R2 \leftarrow R1 = 7$$

Therefore R2 now holds 7 and R1 is still 7. The difference is where the data comes from and goes to: LD and ST each make one memory access, so they take longer than MOV, which stays inside the CPU and needs no memory cycle. In all three cases the value is only copied, not altered.

Data transfer in computer architecture: where it happens

Every data transfer takes place over some path. Inside the CPU it uses the internal buses or a common bus between registers; between the CPU and memory it uses the address, data and control buses; and between the CPU and peripherals it goes through I/O interfaces. The instruction types above are what the programmer sees, while the register transfer language $R2 \leftarrow R1$ is what the hardware does for each of them.

How to write it in the exam

For 5 marks, define data transfer instructions in one sentence (they move data between registers, memory and I/O without changing it), give the table of mnemonics with meanings, and add one line each on load, store and move. If the question is "data transfer and manipulation instructions", add the three groups of data manipulation with their mnemonics, and the sentence that separates the two kinds. A small LD, ST, MOV example earns the last marks.

Addressing modes decide how the operand of these instructions is found; they are covered in Instruction Set and Addressing Modes for BIT students. CSIT students can practise past questions on the CSC213 Computer Architecture page and relate transfers to the Register Transfer and Microoperations unit.

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