Explanation
In many instruction set architectures (ISAs), specific bits within an instruction are designated to represent different components such as registers, immediate values, or operation codes (opcodes). The placement of these bits is determined by the architecture’s design. Let’s break down why the last 3 bits of an instruction might be used to represent a register, such as the second source register (SR2).
An instruction in a typical ISA might be structured as follows:
| Opcode | DR | SR1 | Mode | SR2/Imm5 |
Each field within the instruction has a specific purpose:
- Opcode: Specifies the operation to be performed (e.g., ADD, SUB).
- DR (Destination Register): Specifies the register where the result will be stored.
- SR1 (Source Register 1): Specifies the first source register.
- Mode: Indicates whether the instruction uses an immediate value or a second source register.
- SR2/Imm5: Depending on the mode, this field can either specify the second source register (SR2) or an immediate value (Imm5).
Bit Allocation
In a 16-bit instruction, the bits might be allocated as follows:
| 15-12 | 11-9 | 8-6 | 5 | 4-0 |
| Opcode | DR | SR1 | Mode | SR2/Imm5 |
- Opcode (4 bits): Bits 15-12
- DR (3 bits): Bits 11-9
- SR1 (3 bits): Bits 8-6
- Mode (1 bit): Bit 5
- SR2/Imm5 (5 bits): Bits 4-0
When the mode bit indicates that the instruction uses a second source register (SR2), the last 3 bits (bits 2-0) of the instruction are used to specify SR2. This is because:
- Compact Representation: Using 3 bits allows for 8 possible registers (since 2^3 = 8), which is often sufficient for many architectures.
- Efficient Decoding: Placing SR2 in the last 3 bits allows for quick extraction using bitwise operations.
Example
Consider an instruction instr with the following binary representation:
- Opcode: 1101 (bits 15-12)
- DR: 001 (bits 11-9)
- SR1: 010 (bits 8-6)
- Mode: 1 (bit 5, indicating immediate mode)
- Imm5: 01100 (bits 4-0)
If the mode bit were 0 (indicating register mode), the last 3 bits would represent SR2:
Bitwise Operation
To extract SR2 from the instruction:
uint16_t sr2 = instr & 0x7;
- instr & 0x7 masks the instruction to keep only the last 3 bits.
- 0x7 in binary is 0000 0111, which isolates bits 2-0.
Important points
- The last 3 bits of an instruction are used to represent SR2 because it allows for a compact and efficient way to specify one of 8 possible registers.
- This design choice is common in many ISAs to balance between instruction length and the number of registers.
- Bitwise operations like instr & 0x7 are used to quickly extract these bits for further processing.
- Understanding this bit allocation and extraction process is crucial for working with low-level machine code and assembly language programming.