Control Inputs For Different Instructions - muneeb-mbytes/computerArchitectureCourse GitHub Wiki

R Instruction Type

R-type instruction "add x1, x2, x3", which is a typical arithmetic operation in a RISC (Reduced Instruction Set Computing) architecture, the operation of the datapath can be broken down into four steps:

Instruction Fetch and PC Incrementation:

In the instruction fetch stage, the processor retrieves the instruction from memory. The address from which the instruction is fetched is determined by the Program Counter (PC), which holds the address of the next instruction to be executed. Once the instruction is fetched, the PC is incremented to point to the next instruction in memory. This prepares the PC for fetching the subsequent instruction in the next clock cycle.

Register Read:

In this stage, the values of the source registers specified in the instruction (x2 and x3) are read from the register file. The register file is a small, fast memory component within the CPU that stores the contents of the processor's general-purpose registers. The main control unit is responsible for generating the control signals required for reading the registers from the register file. These control signals include the register addresses (x2 and x3) and the enable signal to activate the read operation.

ALU Operation:

The Arithmetic Logic Unit (ALU) performs the arithmetic operation specified by the opcode of the instruction. In the case of the "add" instruction, the ALU performs addition. The ALU receives the values read from the register file (contents of registers x2 and x3) as input operands. It also receives control signals specifying the ALU operation to be performed (addition) and any additional control signals needed for the operation (e.g., carry-in for addition). The ALU executes the addition operation and produces the result.

Register Write:

In the final stage, the result of the ALU operation (the sum of x2 and x3) is written back to the destination register (x1) in the register file. The control signals generated by the main control unit include the register address (x1) where the result should be written and the enable signal to activate the write operation.

The result is stored in register x1, making it available for future instructions that may require its value. Each of these steps occurs within a single clock cycle. By breaking down the execution of the instruction into these sequential stages, the processor can efficiently process instructions in a pipelined manner, maximizing throughput and performance. This pipelined approach is a key characteristic of modern RISC (Reduced Instruction Set Computing) architectures.

Load Instruction Type

The execution of a load operation, such as the "ld x1, offset(x2)" instruction, into its five steps and provide an explanation for each step:

Instruction Fetch and PC Incrementation:

In the first step, the instruction is fetched from the instruction memory. This involves accessing the memory location specified by the Program Counter (PC), which holds the address of the next instruction to be executed. Simultaneously, the PC is incremented to prepare for fetching the next instruction in the subsequent clock cycle.

Register Read:

During this step, the value of the base register (x2) specified in the instruction is read from the register file. The base register contains the base address or the starting address from which the data will be loaded. The main control unit generates the necessary control signals to enable the read operation from the register file and specifies the register address (x2).

ALU Computation:

The ALU computes the effective memory address by adding the sign-extended 12-bit offset from the instruction to the value read from the register file (x2). This offset is typically a displacement from the base address, allowing the processor to access data at a specific location relative to the base address. Sign extension is performed on the offset to ensure that it is correctly interpreted as a signed value. Sign extension involves extending the most significant bit of the offset to fill the remaining bits, maintaining the integrity of the signed value. The ALU receives the two operands (value from the register file and the sign-extended offset) and performs addition to calculate the effective memory address.

Memory Access:

With the effective memory address computed, the processor accesses the data memory. The memory address points to the location in memory from which data needs to be loaded. Control signals are generated to activate the memory read operation, and the effective memory address calculated in the previous step is used to specify the memory location from which data should be retrieved.

Register Write:

Finally, the data retrieved from the memory is written back into the destination register (x1) in the register file. The main control unit generates control signals to enable the write operation to the register file and specifies the register address (x1) where the data should be stored. The data obtained from the memory is loaded into register x1, making it available for future instructions that may require its value.

Branch-if-equal Instruction Type

Branch-if-equal instruction "beq x1, x2, offset" into its four steps and provide an explanation for each step:

Instruction Fetch and PC Incrementation:

In the first step, the instruction is fetched from the instruction memory. This involves accessing the memory location specified by the Program Counter (PC), which holds the address of the next instruction to be executed. Simultaneously, the PC is incremented to prepare for fetching the next instruction in the subsequent clock cycle.

Register Read:

During this step, the values of the two source registers (x1 and x2) specified in the instruction are read from the register file. These registers hold the data that will be compared to determine whether the branch should be taken. The main control unit generates the necessary control signals to enable the read operation from the register file and specifies the register addresses (x1 and x2).

ALU Subtraction and Branch Target Calculation:

The ALU performs subtraction by subtracting the value of one register (x2) from the value of the other register (x1). This operation determines whether the two values are equal. Simultaneously, the ALU calculates the branch target address by adding the PC value to the sign-extended 12-bit offset from the instruction, left-shifted by one. This offset represents the displacement from the current PC to the branch target address. The result of the subtraction determines the outcome of the branch: if the values are equal (resulting in zero), the branch is taken, and the branch target address is used. Otherwise, the branch is not taken, and the PC remains unchanged (PC + 4).

PC Update Based on ALU Result:

The Zero status information from the ALU is examined to determine whether the branch should be taken or not. If the Zero status indicates that the values are equal (i.e., the subtraction result is zero), the branch is taken, and the branch target address calculated in the previous step is loaded into the PC. This updates the PC to the branch target address, causing the program to jump to the specified location. If the Zero status indicates that the values are not equal (i.e., the subtraction result is non-zero), the branch is not taken, and the PC remains unchanged (PC + 4). This updates the PC to point to the next sequential instruction in memory.

This breakdown illustrates how the branch-if-equal instruction operates, allowing the program to conditionally jump to a different location in the code based on the comparison result of two register values.