RISC-U

A simple library for loading and decoding of ELF64 RISC-U files. This binaries are generated by Selfie, an educational software system of a tiny self-compiling C compiler, a tiny self-executing RISC-V emulator, and a tiny self-hosting RISC-V hypervisor.

RISC-U Instruction Set

RISC-U is a tiny subset of the 64-bit RISC-V instruction set. The selfie system implements a compiler that targets RISC-U as well as a RISC-U emulator that interprets RISC-U code. RISC-U consists of just 14 instructions listed below. For details on the exact encoding, decoding, and semantics of RISC-U code see the selfie implementation.

Machine

A RISC-U machine has a 64-bit program counter denoted pc, 32 general-purpose 64-bit registers numbered 0 to 31 and denoted zero, ra, sp, gp, tp, t0-t2, s0-s1, a0-a7, s2-s11, t3-t6, and 4GB of byte-addressed memory.

Register zero always contains the value 0. Any attempts to update the value in zero are ignored.

Instructions

RISC-U instructions are encoded in 32 bits (4 bytes) each and stored next to each other in memory such that there are two instructions per 64-bit double word. Memory, however, can only be accessed at 64-bit double-word granularity.

The parameters rd, rs1, and rs2 used in the specification of the RISC-U instructions below may denote any of the 32 general-purpose registers.

The parameter imm denotes a signed integer value represented by a fixed number of bits depending on the instruction.

Initialization

lui rd,imm: rd = imm * 2^12; pc = pc + 4 with -2^19 <= imm < 2^19

addi rd,rs1,imm: rd = rs1 + imm; pc = pc + 4 with -2^11 <= imm < 2^11

Memory

ld rd,imm(rs1): rd = memory[rs1 + imm]; pc = pc + 4 with -2^11 <= imm < 2^11

sd rs2,imm(rs1): memory[rs1 + imm] = rs2; pc = pc + 4 with -2^11 <= imm < 2^11

Arithmetic

add rd,rs1,rs2: rd = rs1 + rs2; pc = pc + 4

sub rd,rs1,rs2: rd = rs1 - rs2; pc = pc + 4

mul rd,rs1,rs2: rd = rs1 * rs2; pc = pc + 4

divu rd,rs1,rs2: rd = rs1 / rs2; pc = pc + 4 where rs1 and rs2 are unsigned integers.

remu rd,rs1,rs2: rd = rs1 % rs2; pc = pc + 4 where rs1 and rs2 are unsigned integers.

Comparison

sltu rd,rs1,rs2: if (rs1 < rs2) { rd = 1 } else { rd = 0 } pc = pc + 4 where rs1 and rs2 are unsigned integers.

Control

beq rs1,rs2,imm: if (rs1 == rs2) { pc = pc + imm } else { pc = pc + 4 } with -2^12 <= imm < 2^12 and imm % 2 == 0

jal rd,imm: rd = pc + 4; pc = pc + imm with -2^20 <= imm < 2^20 and imm % 2 == 0

jalr rd,imm(rs1): tmp = ((rs1 + imm) / 2) * 2; rd = pc + 4; pc = tmp with -2^11 <= imm < 2^11

System

ecall: system call number is in a7, parameters are in a0-a2, return value is in a0.

License

Copyright (c) 2020, the Selfie authors. All rights reserved.

Licensed under the MIT license.