---
# System prepended metadata

title: How to Build a SIMD Core on a FPGA?
tags: [FPGA]

---

**1. Understanding SIMD Architecture for FPGA**
SIMD (Single Instruction, Multiple Data) is a parallel processing technique where a single instruction operates on multiple data points simultaneously. For [FPGA](https://www.ampheo.com/c/fpgas-field-programmable-gate-array) implementation, we'll focus on:
![1-s2.0-S1383762115001204-gr1](https://hackmd.io/_uploads/B1lq0rwgxl.jpg)


* Data Parallelism: Same operation applied to multiple data elements
* Vector Processing: Fixed-width operations on packed data
* Scalability: Configurable number of processing elements (PEs)

**2. Core Components of an FPGA SIMD Processor**
**Processing Element (PE) Design**
```
verilog
module processing_element (
    input clk,
    input reset,
    input [31:0] operand_a,
    input [31:0] operand_b,
    input [3:0] opcode,
    output reg [31:0] result
);
    always @(posedge clk or posedge reset) begin
        if (reset) begin
            result <= 32'b0;
        end else begin
            case(opcode)
                4'b0000: result <= operand_a + operand_b;  // ADD
                4'b0001: result <= operand_a - operand_b;  // SUB
                4'b0010: result <= operand_a & operand_b;  // AND
                // Add more operations
                default: result <= 32'b0;
            endcase
        end
    end
endmodule
```

**Vector Register File**
* Implement using Block RAM (BRAM)
* Typically 4-16 vector registers
* Each register holds multiple data elements (e.g., 8x32-bit values)

**Instruction Set Design**
![企业微信截图_20250506164355](https://hackmd.io/_uploads/rk4RsBwlee.png)

**3. Memory Subsystem**
**Data Memory Organization**
* Interleaved memory banks for parallel access
* Use FPGA BRAM resources (e.g., Xilinx's RAMB36E1)
* Example 4-bank memory architecture:

```
Bank 0: Elements 0, 4, 8,...
Bank 1: Elements 1, 5, 9,...
Bank 2: Elements 2, 6, 10,...
Bank 3: Elements 3, 7, 11,...
```
**Memory Controller**
```
verilog
module memory_controller (
    input clk,
    input [31:0] addr,
    input [31:0] wdata,
    input we,
    output [31:0] rdata
);
    // Instantiate multiple BRAMs
    // Implement bank selection logic
    // Handle simultaneous accesses
endmodule
```
**4. Interconnection Network**
**Common Topologies:**
* 1D Mesh: Simple linear connection
* Crossbar: Fully connected (resource intensive)
* Tree: Reduced connectivity (good for reductions)

**Example Crossbar Implementation:**
```
verilog
module crossbar #(
    parameter NUM_PE = 8,
    parameter DATA_WIDTH = 32
)(
    input [NUM_PE*DATA_WIDTH-1:0] inputs,
    input [NUM_PE*$clog2(NUM_PE)-1:0] sel,
    output [NUM_PE*DATA_WIDTH-1:0] outputs
);
    // Implement switching logic
    // Each PE can select any input
endmodule
```

**5. Control Unit Design**
**VLIW (Very Long Instruction Word) Approach**
* Pack multiple operations into wide instructions
* Typical instruction format:

`[Opcode PE0][Opcode PE1]...[Opcode PE7][Operand Addr][Dest Addr]`

**Finite State Machine Example**
```
verilog
module control_unit (
    input clk,
    input reset,
    input [127:0] instruction,
    output reg [3:0] opcode [0:7],
    output reg [31:0] operand_addr,
    output reg [31:0] dest_addr
);
    // Implement fetch-decode-execute cycle
    // Handle instruction pipeline
endmodule
```

**6. Implementation Considerations**
**FPGA Resource Utilization**
* LUTs: For PE logic and control
* DSP Slices: For arithmetic operations
* Block RAM: For vector registers and memory
* Registers: For pipelining and state storage

**Optimization Techniques**
1. Pipelining: Add pipeline registers between stages
2. Time-multiplexing: Share resources when possible
3. Data Alignment: Ensure proper memory alignment
4. Custom Instructions: Add domain-specific operations

**7. Complete SIMD Core Integration**
**Top-Level Module**
```
verilog
module simd_core #(
    parameter NUM_PE = 8,
    parameter DATA_WIDTH = 32
)(
    input clk,
    input reset,
    input [127:0] instruction,
    output [NUM_PE*DATA_WIDTH-1:0] results
);
    // Instantiate all components:
    // - Control unit
    // - Processing elements
    // - Memory subsystem
    // - Interconnect
endmodule
```

**8. Verification and Testing**
**Testbench Structure**
```
verilog
module tb_simd_core;
    // Clock generation
    // Reset generation
    // Instruction stream
    // Memory initialization
    // Result checking
    
    initial begin
        // Test vector addition
        load_memory(0, {32'h1, 32'h2, ..., 32'h8}); // Input A
        load_memory(256, {32'h1, 32'h1, ..., 32'h1}); // Input B
        execute_instruction(VADD, 0, 256, 512); // Add vectors
        verify_results(512, {32'h2, 32'h3, ..., 32'h9});
    end
endmodule
```

**9. Advanced Enhancements**
**Optional Features to Add:**
1. Predication: Conditional execution
2. Masking: Selective element processing
3. Reduction Operations: Sum across vector
4. Scatter/Gather: Irregular memory access
5. Floating-Point Support: Using FPGA [DSPs](https://www.ampheo.com/c/dsp-digital-signal-processors)

**10. Target FPGA Considerations**
**For Xilinx [Spartan-6](https://www.vemeko.com/spartan-6-fpgas/) ([XC6SLX9](https://www.vemeko.com/search.html?keywords=XC6SLX9&method=1)):**
* 16 DSP48A1 slices available
* 576kb block RAM
* Implement 4-8 PEs efficiently

**For Intel [Cyclone 10LP](https://www.vemeko.com/cyclone-10-gx-fpga/):**
* Use M9K memory blocks
* DSP blocks for arithmetic
* Optimize for lower power

This implementation provides a flexible SIMD architecture that can be scaled based on your target [FPGA](https://www.ampheoelec.de/c/fpgas-field-programmable-gate-array)'s resources and your application requirements. The modular design allows for easy customization of the number of processing elements, data width, and supported operations.