Write a note on a simple target machine model.

Simple Target Machine Model

A simple target machine model is used in compilers to generate low-level code such as assembly or machine code. It mimics a typical three-address machine with support for:

• Load/store instructions
• Arithmetic operations
• Conditional and unconditional jumps
• Simple addressing modes

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Machine Characteristics:

• Byte-addressable memory
• n general-purpose registers: R0, R1, ..., Rn-1
• Operands are integers
• Most instructions are of the form:
  OP target, source1, source2

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Instruction Types:

1. Load (LD)

Transfers data from memory to a register.
Syntax: LD dst, addr
Meaning: dst = contents of addr
Examples:

• LD R1, x → Load value at address x into register R1
• LD R1, R2 → Copy value from R2 to R1

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2. Store (ST)

Transfers data from a register to memory.
Syntax: ST r, x
Meaning: x = contents of r
Example: ST R3, a stores the value in R3 into memory location a.

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3. Computation (OP)

Performs arithmetic or logical operations.
Syntax: OP dst, src1, src2
Example:

• ADD R1, R2, R3 → R1 = R2 + R3
• SUB R1, R2, R3 → R1 = R2 - R3
  Supports both binary and unary operations.

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4. Unconditional Jump (BR)

Used for unconditional control transfer.
Syntax: BR L
Effect: Go to instruction labeled L.

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5. Conditional Jump (Bcond)

Used for branching based on conditions.
Syntax: Bcond r, L
Examples:

• BLTZ R1, L → Branch to L if R1 < 0
• BGTZ R2, L → Branch to L if R2 > 0

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Addressing Modes:

1. Direct Addressing:

• LD R1, x → Load value at memory location x into R1.

2. Register Indexed Addressing:

• LD R1, a(R2) → Load value from address a + contents(R2) into R1.
  Used for accessing arrays.

3. Offset Indexed Addressing:

• LD R1, 100(R2) → Load value from address 100 + contents(R2) into R1.
  Used for pointer arithmetic.

4. Indirect Addressing:

• LD R1, *R2 → Load value from address stored in R2
• LD R1, *100(R2) → Load from address stored at 100 + R2
  Used for dereferencing pointers.

5. Immediate Addressing:

• LD R1, #100 → Load constant 100 into R1
• ADD R1, R1, #5 → Add 5 to the value in R1

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Example:

LD R1, #10         // Load 10 into R1
ST R1, a           // Store R1 into memory location a
LD R2, a(R3)       // Load value from address a + R3
ADD R4, R2, #5     // R4 = R2 + 5
BLTZ R4, NEG       // If R4 < 0, jump to NEG