Copyright (c) 2015-2018, the Selfie Project authors. All rights reserved. Please see the AUTHORS file for details. Use of this source code is governed by a BSD license that can be found in the LICENSE file.
Selfie is a project of the Computational Systems Group at the Department of Computer Sciences of the University of Salzburg in Austria. For further information and code please refer to:
http://selfie.cs.uni-salzburg.at
This document provides an overview of the 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.
A RISC-U machine has a 64-bit program counter denoted $pc, 32 general-purpose 64-bit registers ($zero, $ra, $sp, $gp, $tp, $t0-$t2, $fp, $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.
RISC-U instructions are 32-bit, two 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 RISC-U instructions 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.
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
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
remu $rd,$rs1,$rs2: $rd = $rs1 % $rs2; $pc = $pc + 4
sltu $rd,$rs1,$rs2: if ($rs1 < $rs2) { $rd = 1 } else { $rd = 0 } $pc = $pc + 4
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
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
ecall: system call number is in $a7, parameters are in $a0-$a2, return value is in $a0.