Designing the WRAMP

1. Designing the WRAMP Dean Armstrong The University of Waikato

2. Aims and Objectives • Provide a suitable hardware platform for teaching Computer Systems • Commercial processors generally not suitable for teaching • Designed for high performance/low cost • Often have quirks that complicate teaching • Need to design a custom processor

3. Implementing a Processor • Could use full custom design, but… • Extremely expensive • Long design cycle • Inflexible • Another alternative is to use ‘Reconfigurable Logic’ • General purpose chip which can be configured to act in a certain way

4. Reconfigurable Logic • Comparatively inexpensive • Particularly in small quantities • Short design cycle • Extremely flexible

5. FPGA’s • Field Programmable Gate Arrays (FPGA’s) are a kind of Reconfigurable Logic • Static RAM based • An ‘FPGA Image’ is loaded to specify how the chip is to act • When the power is off the image is lost • An FPGA is used for the WRAMP processor on the REX board

6. Creating an FPGA Image • Can use VHDL • VHDL = VHSIC Hardware Description Language • VHSIC = Very High Speed Integrated Circuit • VHDL is like a programming language • It allows hardware to be described

7. VHDL code is synthesised to create an FPGA image Code can also be simulated to test its correctness a_bus <= pc when pc_out = ‘1’ else (other => ‘Z’); ... process (reset, clk) begin if reset = ‘1’ then pc <= (others => ‘0’); elsif rising_edge(clk) then if pc_in = ‘1’ then pc <= c_bus; end if; end if; end process; VHDL example code

8. Processor Design Choices • Need to consider style of architecture • RISC/CISC • Memory and I/O models • Number of registers • Choice of instructions • Exception mechanism

9. WRAMP • RISC style processor • Load/store architecture • 16 general purpose registers • Consistent and predictable instruction set • Simple exception mechanism

10. Processor Design • Firstly construct a datapath which will allow execution of all the instructions • This is a matter of drawing block diagrams • Decide on instruction encodings • Design control logic to perform the operations • Draw state machines

11. Datapath Design • We use building blocks such as • Registers • Sometimes combined as a Register File • ALU • Buses • Memory Interface • When combining these we need to… • Ensure that we can execute all the instructions • Keep the datapath as simple as possible

12. The WRAMP Datapath Memory Interface Register File Temp ALU PC IR

13. Instruction Encodings

14. Example Control Insn Fetch PC Incr OPcode = “0000” OPcode = “0110” ALU op OPcode = “0100” Save $ra Do Jump

15. Control Signals • We need to decide which control signals should be set in each state. Eg. • Insn Fetch • pc_out, ir_in, mem_read • PC Incr • pc_out, alu_out, alu_func = inc, pc_in • Save $ra • pc_out, b_out, sel_b = $0, alu_out, alu_func = add, c_in, sel_c = $ra • Do jump • imm_20_out, a_out, sel_a = $0, alu_out, alu_func = add, pc_in

16. Processor Implementation • With the datapath and control decided we can begin implementation in VHDL • We can run the code on a simulator to ensure that it works correctly

17. Supporting Tools • To allow use of the WRAMP, certain tools were created • Assembler (wasm) • Linker (wlink) • C Compiler (wcc) • We also needed a mini operating system to run on the boards (WRAMPmon)

18. Schematic Design

19. PCB Layout