HF-RISC - 32-bit 3-stage Pipelined MIPS / RISC-V Microcontroller

HF-RISC is a small 32-bit, in order, 3-stage pipelined MIPS / RISC-V microcontroller designed at the Embedded Systems Group (GSE) of the Faculty of Informatics, PUCRS, Brazil. All registers / memory accesses are synchronized to the rising edge of clock. The core can be easily integrated into several applications, and interfaces directly to standard synchronous memories. Pipeline stages are:

• FETCH: Instruction memory is accessed (address is PC), data becomes available in one cycle. PC is updated.
• DECODE: An instruction is fed into the decoding / control logic and values are registered for the next stage. Pipeline stalls, as well as bubble insertion is performed in this stage.
• EXECUTE: The register file is accessed and the ALU calculates the result. Data access is performed (loads and stores) or simply the result (or pc) is written to the register file (normal operations). Branch target and its outcome are calculated.

• Memory is accessed in big endian mode.
• No unaligned loads/stores.
• No co-processor is implemented and all peripherals are memory mapped.
• Loads and stores take 3 cycles. The processor datapath is organized as a Von Neumann machine, so there is only one memory interface that is shared betweeen code and data accesses. No load delay slots are needed in code.
• Branches are performed on a single cycle (not taken) or 3 cycles (taken), including two branch delay slots. This is a side effect of the pipeline refill and memory access policy. All other instructions are single cycle. The first delay slot can be filled with an instruction, reducing the cost to 2 cycles. The second delay slot is completely useless and the instruction in this slot is discarded. No branch predictor is implemented (default branch target is 'not taken'). Minor modifications in the datapath can turn the second branch delay slot usable, but the current toolchain isn't compatible with this behavior, so a bubble is inserted.
• Interrupts are handled using memory mapped VECTOR, CAUSE, MASK, STATUS and EPC registers. The VECTOR register is used to hold the address of the default (non-vectored) interrupt handler. The CAUSE register is read only and peripheral interrupt lines are connected to this register. The MASK register is read/write and holds the interrupt mask for the CAUSE register. The interrupt STATUS register is automatically cleared on interrupts, and is set by software when returning from interrupts - this works as a global interrupt enable/disable flag. This register is read and write capable, so it can also be cleared by software. Setting this register on return from interrupts re-enables interrupts after a few cycles. The EPC register holds the program counter when the processor is interrupted (we should re-execute the last instruction (EPC-4), as it was not commited yet). EPC is a read only register, and is used to return from an interrupt using simple LW / JR instructions. As an interrupt is accepted, the processor jumps to VECTOR address where the first level of irq handling is done. A second level handler (in C) implements the interrupt priority mechanism and calls the appropriate ISR for each interrupt.
• Built in peripherals: running counter (32 bit), two counter comparators (32 and 24 bit), I/O ports and UART. The UART baud rate is defined in a 16 bit divisor register. Two counter bits (bits 18 and 16 and their complements) are tied to interrupt lines, so are the two counter comparators and the UART.
• The core pipeline can be stalled by external logic, allowing the design to be integrated with a more complex memory system, such as DRAM controllers and caches.

Compiler Support

Patched GNU GCC version 4.9.3. Mandatory compiler options (for bare metal) are: -mips1(**) -mpatfree -mfix-r4000 -mno-check-zero-division -msoft-float -fshort-double -ffreestanding -nostdlib -fomit-frame-pointer -G 0 -mnohwmult -mnohwdiv -ffixed-lo -ffixed-hi.

A complete set of flags to be used with the compiler is provided in example applications, along with a small C library. A full compiler toolchain (including Newlib) can be generated using the provided scripts.

(**) "-mips2 -mno-branch-likely" can be used instead of "-mips1". The result is similar to the code generated with "-mips1", but no useless nops are inserted after loads on data hazards (load delay slots).

Instruction Set

The following instructions from the MIPS I instruction set were implemented (41 opcodes):

• Arithmetic Instructions: addiu, addu, subu
• Logic Instructions: and, andi, nor, or, ori, xor, xori
• Shift Instructions: sll, sra, srl, sllv, srav, srlv
• Comparison Instructions: slt, sltu, slti, sltiu
• Load/Store Instructions: lui, lb, lbu, lh, lhu, lw, sb, sh, sw
• Branch Instructions: beq, bne, bgez, bgezal, bgtz, blez, bltzal, bltz
• Jump Instructions: j, jal, jr, jalr

Memory Map

The memory map is separated into regions.

• ROM: 0x00000000 - 0x1fffffff (512MB)
• System 0x20000000 - 0x3fffffff (512MB)
• SRAM 0x40000000 - 0x5fffffff (512MB)
• External RAM / device 0x60000000 - 0x9fffffff (1GB)
• External RAM / device 0xa0000000 - 0xdfffffff (1GB) (uncached)
• External Peripheral 0xe0000000 - 0xefffffff (256MB) (uncached)
• Peripheral (core) 0xf0000000 - 0xf7ffffff (128MB) (uncached)
• Peripheral (extended) 0xf8000000 - 0xffffffff (128MB) (uncached)

• IRQ_VECTOR: 0xf0000000
• IRQ_CAUSE: 0xf0000010
• IRQ_MASK: 0xf0000020
• IRQ_STATUS: 0xf0000030
• IRQ_EPC: 0xf0000040
• COUNTER: 0xf0000050
• COMPARE: 0xf0000060
• COMPARE2: 0xf0000070
• EXTIO_IN: 0xf0000080
• EXTIO_OUT: 0xf0000090
• DEBUG: 0xf00000d0
• UART_WRITE / UART_READ: 0xf00000e0
• UART_DIVISOR: 0xf00000f0

HF-RISCV

Another version of this core implements the RV32I base user level instruction set of the RISC-V architecture. The core organization is basically the same as HF-RISC, including the memory map and software compatibility (a given application just has to be recompiled to the RV32I target). Improvements (compared to HF-RISC) include shorter critical path for higher clock frequency and an exception handling mechanism (traps) for unimplemented opcodes.

Other advantages of this core are related to the ISA, which improves a lot (compared to MIPS) several aspects: better code density (even without the compressed code extensions), more extensible ISA, no delayed load/branches, better immediate fields encoding and nice support for relocatable code. Also, there is no need to tweak the compiler (as done on the HF-RISC) because RV32I is as simple as it gets.

The toolchain used with this core is the GCC compiler with a backend for the RISC-V architecture. Sources and installation instructions can be found at https://github.com/riscv/riscv-gnu-toolchain.

Performance

HF-RISC: 0.96 CoreMark/MHz
HF-RISCV: 0.84 CoreMark/MHz*

If a multiplier is used (not included in this design) the score is around 2.1 Coremark/MHz.

*HF-RISCV doesn't include the multiply/divide (M) extension (pure RV32I) and doesn't include dalayed branches as found on MIPS. This sacrifices performance a bit, but simplifies the ISA for deeper pipelines and OOO execution. Based on several experiments, the inclusion of the M module will boost the performance of this core beyond the HF-RISC version, even without branch prediction.