Montreal
========
:revnumber: 0.1.0-draft

A RISC-V RV32E processor designed for TinyTapeout.

RV32E
-----

https://docs.riscv.org/reference/isa/unpriv/unpriv-index.html[ISA Reference]

Reduces number of integer registers to 16 general-purpose registers
(x0-x15). Upper 16 registers consume around one quarter of the total
area of the core excluding memories, thus their removal saves around 25%
core area with a corresponding core power reduction.

Each register is 32 bits wide.

[cols="1,2,4", options="header"]
|===

| Register
| Description
| Purpose

| x0
| dedicated zero
| Source operand for value 0.

| x1
| return address
| Written by JAL/JALR. Holds address to be returned to once a function returns.
Must be saved to stack before calling another function.

| x2
| stack pointer
| Always points to top of the stack in RAM. Decremented to allocate space,
incremented to free it. CALLEE must restore it BEFORE returning.

| x3
| global pointer
|

| x4
| thread pointer
|

| x5
| alternate link reg
| Second link register for nested JAL sequences. Caller must save
if needed across a call.

| x6
| general scratch reg
|

| x7
| general scratch reg
|

| x8
| frame pointer
|

| x9
| saved register
|

| x10
| first function argument
| Also used to return integer results from function.

| x11
| second function argument
|

| x12
| third function argument
|

| x13
| fourth function argument
|

| x14
| fifth function argument
|

| x15
| sixth function argument
|

| pc
| program counter
| Address of current instruction

|===

Instruction Set
---------------

.Relevant opcodes for the IP. `inst[1:0] = 11`.
[cols="2*^1m,8*^2m", hrows=2]
|===

|
|
8+^| inst[4:2]

|
|
| 000
| 001
| 010
| 011
| 100
| 101
| 110
| 111

1.4+.^| inst[6:5]
| 00
| LOAD
|
|
| MISC-MEM
| OP-IMM
| AUIPC
|
|

| 01
| STORE
|
|
|
| OP
| LUI
|
|

| 10
|
|
|
|
|
|
|
|

| 11
| BRANCH
| JALR
|
| JAL
| SYSTEM
|
|
|
|===

=== Immediate Types

The labels `[7]`, `[20]`, and `[31]` represent the bits `inst[7]`, `inst[20]`, and `inst[31]`, respectively.

image::docs/images/i-immediate.svg[width=100%]
image::docs/images/s-immediate.svg[width=100%]
image::docs/images/b-immediate.svg[width=100%]
image::docs/images/u-immediate.svg[width=100%]
image::docs/images/j-immediate.svg[width=100%]

=== LUI

image::docs/images/lui.svg[width=100%]

=== AUIPC

image::docs/images/auipc.svg[width=100%]

=== OP-IMM

image::docs/images/addi.svg[width=100%]
image::docs/images/slti.svg[width=100%]
image::docs/images/sltiu.svg[width=100%]
image::docs/images/xori.svg[width=100%]
image::docs/images/ori.svg[width=100%]
image::docs/images/andi.svg[width=100%]
image::docs/images/slli.svg[width=100%]
image::docs/images/srli.svg[width=100%]
image::docs/images/srai.svg[width=100%]

=== OP

image::docs/images/add.svg[width=100%]
image::docs/images/sub.svg[width=100%]
image::docs/images/slt.svg[width=100%]
image::docs/images/sltu.svg[width=100%]
image::docs/images/xor.svg[width=100%]
image::docs/images/or.svg[width=100%]
image::docs/images/and.svg[width=100%]
image::docs/images/sll.svg[width=100%]
image::docs/images/srl.svg[width=100%]
image::docs/images/sra.svg[width=100%]
image::docs/images/czero.eqz.svg[width=100%]
image::docs/images/czero.nez.svg[width=100%]

=== JAL

image::docs/images/jal.svg[width=100%]

=== JALR

image::docs/images/jalr.svg[width=100%]

=== BRANCH

The labels `[11]` and `[12]` represent the bits `imm[11]` and `imm[12]` respectively.

image::docs/images/beq.svg[width=100%]
image::docs/images/bne.svg[width=100%]
image::docs/images/blt.svg[width=100%]
image::docs/images/bge.svg[width=100%]
image::docs/images/bltu.svg[width=100%]
image::docs/images/bgeu.svg[width=100%]

=== LOAD

image::docs/images/lb.svg[width=100%]
image::docs/images/lh.svg[width=100%]
image::docs/images/lw.svg[width=100%]
image::docs/images/lbu.svg[width=100%]
image::docs/images/lhu.svg[width=100%]

=== STORE

image::docs/images/sb.svg[width=100%]
image::docs/images/sh.svg[width=100%]
image::docs/images/sw.svg[width=100%]

=== MISC-MEM

image::docs/images/fence.svg[width=100%]

=== SYSTEM

image::docs/images/ecall.svg[width=100%]
image::docs/images/ebreak.svg[width=100%]