---
title: "Fyntv Instruction Set Architecture"
subtitle: "Reference Manual"
version: "0.0.0.1"
date: "June 2026"
status: "Draft"
geometry: margin=1in
toc: true
numbersections: true
---

\newpage

# Revision History

| Version | Date | Description |
|---------|------|-------------|
| 0.0.0.1 | June 2026 | Initial Fyntv release |

\newpage

\tableofcontents

\newpage

# Introduction

This document defines the **Fyntv Instruction Set Architecture (ISA)** version 0.0.0.1.

## Scope

This specification covers:

- Instruction formats and encoding
- Register file and calling convention
- All base and extension instructions
- Control and status registers (CSRs)
- Exception and trap handling

## ISA Overview

The Fyntv ISA is a load-store architecture with:

- 32 general-purpose registers (x0–x31)
- Fixed-width 32-bit instructions
- Three operand formats: register (R-type), immediate (I-type), and store (S-type)
- Branch (B-type) and upper-immediate (U-type) variants
- A separate jump-and-link (J-type) format

\newpage

# Registers

## General-Purpose Registers

The architecture provides 32 general-purpose registers (x0–x31), each 32 bits wide. Register x0 is hardwired to the constant zero.

| Register | ABI Name | Description | Callee-Saved |
|----------|----------|-------------|--------------|
| x0 | zero | Always-zero register — reads return 0, writes are discarded | — |
| x1 | ra | Return address link register | No |
| x2 | sp | Stack pointer | Yes |
| x3 | gp | Global pointer | Yes |
| x4 | tp | Thread pointer | Yes |
| x5 | t0 | Temporary register 0 | No |
| x6 | t1 | Temporary register 1 | No |
| x7 | t2 | Temporary register 2 | No |
| x8 | s0 | Saved register 0 / frame pointer | Yes |
| x9 | s1 | Saved register 1 | Yes |
| x10 | a0 | Function argument 0 / return value 0 | No |
| x11 | a1 | Function argument 1 / return value 1 | No |
| x12 | a2 | Function argument 2 | No |
| x13 | a3 | Function argument 3 | No |
| x14 | a4 | Function argument 4 | No |
| x15 | a5 | Function argument 5 | No |
| x16 | a6 | Function argument 6 | No |
| x17 | a7 | Function argument 7 | No |
| x18 | s2 | Saved register 2 | Yes |
| x19 | s3 | Saved register 3 | Yes |
| x20 | s4 | Saved register 4 | Yes |
| x21 | s5 | Saved register 5 | Yes |
| x22 | s6 | Saved register 6 | Yes |
| x23 | s7 | Saved register 7 | Yes |
| x24 | s8 | Saved register 8 | Yes |
| x25 | s9 | Saved register 9 | Yes |
| x26 | s10 | Saved register 10 | Yes |
| x27 | s11 | Saved register 11 | Yes |
| x28 | t3 | Temporary register 3 | No |
| x29 | t4 | Temporary register 4 | No |
| x30 | t5 | Temporary register 5 | No |
| x31 | t6 | Temporary register 6 | No |

### ABI Calling Convention

| Register | ABI Name | Caller-Saved | Argument |
|----------|----------|-------------|----------|
| x0 | zero | No | No |
| x1 | ra | No | No |
| x2 | sp | No | No |
| x3 | gp | No | No |
| x4 | tp | No | No |
| x5 | t0 | Yes | No |
| x6 | t1 | Yes | No |
| x7 | t2 | Yes | No |
| x8 | s0 | No | No |
| x9 | s1 | No | No |
| x10 | a0 | Yes | Yes |
| x11 | a1 | Yes | Yes |
| x12 | a2 | Yes | Yes |
| x13 | a3 | Yes | Yes |
| x14 | a4 | Yes | Yes |
| x15 | a5 | Yes | Yes |
| x16 | a6 | Yes | Yes |
| x17 | a7 | Yes | Yes |
| x18 | s2 | No | No |
| x19 | s3 | No | No |
| x20 | s4 | No | No |
| x21 | s5 | No | No |
| x22 | s6 | No | No |
| x23 | s7 | No | No |
| x24 | s8 | No | No |
| x25 | s9 | No | No |
| x26 | s10 | No | No |
| x27 | s11 | No | No |
| x28 | t3 | Yes | No |
| x29 | t4 | Yes | No |
| x30 | t5 | Yes | No |
| x31 | t6 | Yes | No |

\newpage

# Instruction Formats

Instructions are encoded in one of several fixed formats. All formats share the same opcode placement (bits 6:0) to allow efficient decoding.

### R-Type Format

Bit width: **32 bits**

```
 31:25 |24:20|19:15|14:12|11:7| 6:0 
funct7 | rs2 | rs1 |funct3| rd  |opcode 
   7   |  5  |  5  | 3 |  5  |   7   
```

### I-Type Format

Bit width: **32 bits**

```
   31:20    |19:15|14:12|11:7| 6:0 
    imm     | rs1 |funct3| rd  |opcode 
     12     |  5  | 3 |  5  |   7   
```

### S-Type Format

Bit width: **32 bits**

```
 31:25 |24:20|19:15|14:12|11:7| 6:0 
imm_hi | rs2 | rs1 |funct3|imm_lo|opcode 
   7   |  5  |  5  | 3 |  5  |   7   
```

### B-Type Format

Bit width: **32 bits**

```
31:31|30:25 |24:20|19:15|14:12|11:7| 6:0 
imm_12|imm_10_5| rs2 | rs1 |funct3|imm_4_1|opcode 
1|  6   |  5  |  5  | 3 |  5  |   7   
```

### U-Type Format

Bit width: **32 bits**

```
       31:12        |11:7| 6:0 
        imm         | rd  |opcode 
         20         |  5  |   7   
```

### J-Type Format

Bit width: **32 bits**

```
31:31|  30:21   |20:20| 19:12  |11:7| 6:0 
imm_20| imm_10_1 |imm_11|imm_19_12| rd  |opcode 
1|    10    |1|   8    |  5  |   7   
```

\newpage

# Instruction Set

## Branch Operations

| Mnemonic | Format | Opcode | Funct3 | Funct7 | Operands | Description |
|----------|--------|--------|--------|--------|----------|-------------|
| `BNE` | B | `0x63` | 0x1 | — | rs1, rs2, label | Branch not equal |
| `BEQ` | B | `0x63` | 0x0 | — | rs1, rs2, label | Branch equal |
| `BLT` | B | `0x63` | 0x4 | — | rs1, rs2, label | Branch less than (signed) |
| `BGE` | B | `0x63` | 0x5 | — | rs1, rs2, label | Branch greater than or equal (signed) |
| `BLTU` | B | `0x63` | 0x6 | — | rs1, rs2, label | Branch less than (unsigned) |
| `BGEU` | B | `0x63` | 0x7 | — | rs1, rs2, label | Branch greater than or equal (unsigned) |
## Integer Operations

| Mnemonic | Format | Opcode | Funct3 | Funct7 | Operands | Description |
|----------|--------|--------|--------|--------|----------|-------------|
| `AUIPC` | U | `0x17` | — | — | rd, imm20 | Add upper immediate to PC — forms PC-relative address |
| `SLT` | R | `0x33` | 0x2 | 0x00 | rd, rs1, rs2 | Set if rs1 is less than rs2 (signed) |
| `SLTU` | R | `0x33` | 0x3 | 0x00 | rd, rs1, rs2 | Set if rs1 is less than rs2 (unsigned) |
| `SLTI` | I | `0x13` | 0x2 | — | rd, rs1, imm12 | Set if rs1 is less than immediate (signed) |
| `SLTIU` | I | `0x13` | 0x3 | — | rd, rs1, imm12 | Set if rs1 is less than immediate (unsigned) |
| `LUI` | U | `0x37` | — | — | rd, imm20 | Load upper immediate — places 20-bit immediate in upper 20 bits of rd |
| `ADD` | R | `0x33` | 0x0 | 0x00 | rd, rs1, rs2 | Add registers |
| `SUB` | R | `0x33` | 0x0 | 0x20 | rd, rs1, rs2 | Subtract registers |
| `ADDI` | I | `0x13` | 0x0 | — | rd, rs1, imm12 | Add sign-extended 12-bit immediate to register rs1 |
## Jump Operations

| Mnemonic | Format | Opcode | Funct3 | Funct7 | Operands | Description |
|----------|--------|--------|--------|--------|----------|-------------|
| `JAL` | J | `0x6F` | — | — | rd, label | Jump and link — jump to PC+offset, save return address to rd |
| `JALR` | I | `0x67` | 0x0 | — | rd, rs1, offset | Jump and link register — jump to rs1+offset, save return address |
## Logical Operations

| Mnemonic | Format | Opcode | Funct3 | Funct7 | Operands | Description |
|----------|--------|--------|--------|--------|----------|-------------|
| `XOR` | R | `0x33` | 0x4 | 0x00 | rd, rs1, rs2 | Bitwise XOR |
| `OR` | R | `0x33` | 0x6 | 0x00 | rd, rs1, rs2 | Bitwise OR |
| `AND` | R | `0x33` | 0x7 | 0x00 | rd, rs1, rs2 | Bitwise AND |
| `ORI` | I | `0x13` | 0x6 | — | rd, rs1, imm12 | Bitwise OR with immediate |
| `XORI` | I | `0x13` | 0x4 | — | rd, rs1, imm12 | Bitwise XOR with immediate |
| `ANDI` | I | `0x13` | 0x7 | — | rd, rs1, imm12 | Bitwise AND with immediate |
## Memory Operations

| Mnemonic | Format | Opcode | Funct3 | Funct7 | Operands | Description |
|----------|--------|--------|--------|--------|----------|-------------|
| `SW` | S | `0x23` | 0x2 | — | rs2, offset(rs1) | Store word |
| `LB` | I | `0x03` | 0x0 | — | rd, offset(rs1) | Load byte (sign-extended) |
| `LH` | I | `0x03` | 0x1 | — | rd, offset(rs1) | Load halfword (sign-extended) |
| `LW` | I | `0x03` | 0x2 | — | rd, offset(rs1) | Load word |
| `LBU` | I | `0x03` | 0x4 | — | rd, offset(rs1) | Load byte (zero-extended) |
| `LHU` | I | `0x03` | 0x5 | — | rd, offset(rs1) | Load halfword (zero-extended) |
| `SB` | S | `0x23` | 0x0 | — | rs2, offset(rs1) | Store byte |
| `SH` | S | `0x23` | 0x1 | — | rs2, offset(rs1) | Store halfword |
## Shift Operations

| Mnemonic | Format | Opcode | Funct3 | Funct7 | Operands | Description |
|----------|--------|--------|--------|--------|----------|-------------|
| `SRAI` | I | `0x13` | 0x5 | — | rd, rs1, shamt5 | Arithmetic right shift by immediate shift amount |
| `SLLI` | I | `0x13` | 0x1 | — | rd, rs1, shamt5 | Logical left shift by immediate shift amount |
| `SRA` | R | `0x33` | 0x5 | 0x20 | rd, rs1, rs2 | Arithmetic right shift by lower 5 bits of rs2 |
| `SRL` | R | `0x33` | 0x5 | 0x00 | rd, rs1, rs2 | Logical right shift by lower 5 bits of rs2 |
| `SLL` | R | `0x33` | 0x1 | 0x00 | rd, rs1, rs2 | Logical left shift by lower 5 bits of rs2 |
| `SRLI` | I | `0x13` | 0x5 | — | rd, rs1, shamt5 | Logical right shift by immediate shift amount |
## Synchronization Operations

| Mnemonic | Format | Opcode | Funct3 | Funct7 | Operands | Description |
|----------|--------|--------|--------|--------|----------|-------------|
| `FENCEI` | I | `0x0F` | 0x1 | — | — | Instruction fence — synchronizes instruction and data streams |
| `FENCE` | I | `0x0F` | 0x0 | — | pred, succ | Memory ordering fence |
## System Operations

| Mnemonic | Format | Opcode | Funct3 | Funct7 | Operands | Description |
|----------|--------|--------|--------|--------|----------|-------------|
| `CSRRCI` | I | `0x73` | 0x7 | — | rd, csr, uimm5 | Atomic read and clear bits in CSR with immediate |
| `ECALL` | I | `0x73` | 0x0 | — | — | Environment call — raises a system call exception |
| `EBREAK` | I | `0x73` | 0x0 | — | — | Breakpoint — raises a breakpoint exception |
| `CSRRW` | I | `0x73` | 0x1 | — | rd, csr, rs1 | Atomic read/write CSR — writes rs1 to CSR, returns old value in rd |
| `CSRRS` | I | `0x73` | 0x2 | — | rd, csr, rs1 | Atomic read and set bits in CSR |
| `CSRRC` | I | `0x73` | 0x3 | — | rd, csr, rs1 | Atomic read and clear bits in CSR |
| `CSRRWI` | I | `0x73` | 0x5 | — | rd, csr, uimm5 | Atomic read/write CSR with immediate |
| `CSRRSI` | I | `0x73` | 0x6 | — | rd, csr, uimm5 | Atomic read and set bits in CSR with immediate |

\newpage

## Instruction Details

### BNE

- **Format:** B
- **Opcode:** `0x63`
- **Funct3:** `0x1`
- **Operands:** `rs1, rs2, label`
- **Description:** Branch not equal
- **Operation:** `if (rs1 != rs2) PC += sext(offset)`

### BEQ

- **Format:** B
- **Opcode:** `0x63`
- **Funct3:** `0x0`
- **Operands:** `rs1, rs2, label`
- **Description:** Branch equal
- **Operation:** `if (rs1 == rs2) PC += sext(offset)`

### BLT

- **Format:** B
- **Opcode:** `0x63`
- **Funct3:** `0x4`
- **Operands:** `rs1, rs2, label`
- **Description:** Branch less than (signed)
- **Operation:** `if (rs1 < rs2) PC += sext(offset)`

### BGE

- **Format:** B
- **Opcode:** `0x63`
- **Funct3:** `0x5`
- **Operands:** `rs1, rs2, label`
- **Description:** Branch greater than or equal (signed)
- **Operation:** `if (rs1 >= rs2) PC += sext(offset)`

### BLTU

- **Format:** B
- **Opcode:** `0x63`
- **Funct3:** `0x6`
- **Operands:** `rs1, rs2, label`
- **Description:** Branch less than (unsigned)
- **Operation:** `if (rs1 < rs2) PC += sext(offset)`

### BGEU

- **Format:** B
- **Opcode:** `0x63`
- **Funct3:** `0x7`
- **Operands:** `rs1, rs2, label`
- **Description:** Branch greater than or equal (unsigned)
- **Operation:** `if (rs1 >= rs2) PC += sext(offset)`

### AUIPC

- **Format:** U
- **Opcode:** `0x17`
- **Operands:** `rd, imm20`
- **Description:** Add upper immediate to PC — forms PC-relative address
- **Operation:** `rd = PC + (imm20 << 12)`

### SLT

- **Format:** R
- **Opcode:** `0x33`
- **Funct3:** `0x2`
- **Funct7:** `0x00`
- **Operands:** `rd, rs1, rs2`
- **Description:** Set if rs1 is less than rs2 (signed)
- **Operation:** `rd = (rs1 < rs2) ? 1 : 0`

### SLTU

- **Format:** R
- **Opcode:** `0x33`
- **Funct3:** `0x3`
- **Funct7:** `0x00`
- **Operands:** `rd, rs1, rs2`
- **Description:** Set if rs1 is less than rs2 (unsigned)
- **Operation:** `rd = (rs1 < rs2) ? 1 : 0`

### SLTI

- **Format:** I
- **Opcode:** `0x13`
- **Funct3:** `0x2`
- **Operands:** `rd, rs1, imm12`
- **Description:** Set if rs1 is less than immediate (signed)
- **Operation:** `rd = (rs1 < sext(imm12)) ? 1 : 0`

### SLTIU

- **Format:** I
- **Opcode:** `0x13`
- **Funct3:** `0x3`
- **Operands:** `rd, rs1, imm12`
- **Description:** Set if rs1 is less than immediate (unsigned)
- **Operation:** `rd = (rs1 < sext(imm12)) ? 1 : 0`

### LUI

- **Format:** U
- **Opcode:** `0x37`
- **Operands:** `rd, imm20`
- **Description:** Load upper immediate — places 20-bit immediate in upper 20 bits of rd
- **Operation:** `rd = imm20 << 12`

### ADD

- **Format:** R
- **Opcode:** `0x33`
- **Funct3:** `0x0`
- **Funct7:** `0x00`
- **Operands:** `rd, rs1, rs2`
- **Description:** Add registers
- **Operation:** `rd = rs1 + rs2`

### SUB

- **Format:** R
- **Opcode:** `0x33`
- **Funct3:** `0x0`
- **Funct7:** `0x20`
- **Operands:** `rd, rs1, rs2`
- **Description:** Subtract registers
- **Operation:** `rd = rs1 - rs2`

### ADDI

- **Format:** I
- **Opcode:** `0x13`
- **Funct3:** `0x0`
- **Operands:** `rd, rs1, imm12`
- **Description:** Add sign-extended 12-bit immediate to register rs1
- **Operation:** `rd = rs1 + sext(imm12)`

### JAL

- **Format:** J
- **Opcode:** `0x6F`
- **Operands:** `rd, label`
- **Description:** Jump and link — jump to PC+offset, save return address to rd
- **Operation:** `rd = PC + 4; PC += sext(offset)`

### JALR

- **Format:** I
- **Opcode:** `0x67`
- **Funct3:** `0x0`
- **Operands:** `rd, rs1, offset`
- **Description:** Jump and link register — jump to rs1+offset, save return address
- **Operation:** `rd = PC + 4; PC = (rs1 + sext(offset)) & ~1`

### XOR

- **Format:** R
- **Opcode:** `0x33`
- **Funct3:** `0x4`
- **Funct7:** `0x00`
- **Operands:** `rd, rs1, rs2`
- **Description:** Bitwise XOR
- **Operation:** `rd = rs1 ^ rs2`

### OR

- **Format:** R
- **Opcode:** `0x33`
- **Funct3:** `0x6`
- **Funct7:** `0x00`
- **Operands:** `rd, rs1, rs2`
- **Description:** Bitwise OR
- **Operation:** `rd = rs1 | rs2`

### AND

- **Format:** R
- **Opcode:** `0x33`
- **Funct3:** `0x7`
- **Funct7:** `0x00`
- **Operands:** `rd, rs1, rs2`
- **Description:** Bitwise AND
- **Operation:** `rd = rs1 & rs2`

### ORI

- **Format:** I
- **Opcode:** `0x13`
- **Funct3:** `0x6`
- **Operands:** `rd, rs1, imm12`
- **Description:** Bitwise OR with immediate
- **Operation:** `rd = rs1 | sext(imm12)`

### XORI

- **Format:** I
- **Opcode:** `0x13`
- **Funct3:** `0x4`
- **Operands:** `rd, rs1, imm12`
- **Description:** Bitwise XOR with immediate
- **Operation:** `rd = rs1 ^ sext(imm12)`

### ANDI

- **Format:** I
- **Opcode:** `0x13`
- **Funct3:** `0x7`
- **Operands:** `rd, rs1, imm12`
- **Description:** Bitwise AND with immediate
- **Operation:** `rd = rs1 & sext(imm12)`

### SW

- **Format:** S
- **Opcode:** `0x23`
- **Funct3:** `0x2`
- **Operands:** `rs2, offset(rs1)`
- **Description:** Store word
- **Operation:** `MEM[rs1 + offset][31:0] = rs2[31:0]`

### LB

- **Format:** I
- **Opcode:** `0x03`
- **Funct3:** `0x0`
- **Operands:** `rd, offset(rs1)`
- **Description:** Load byte (sign-extended)
- **Operation:** `rd = sext(MEM[rs1 + offset][7:0])`

### LH

- **Format:** I
- **Opcode:** `0x03`
- **Funct3:** `0x1`
- **Operands:** `rd, offset(rs1)`
- **Description:** Load halfword (sign-extended)
- **Operation:** `rd = sext(MEM[rs1 + offset][15:0])`

### LW

- **Format:** I
- **Opcode:** `0x03`
- **Funct3:** `0x2`
- **Operands:** `rd, offset(rs1)`
- **Description:** Load word
- **Operation:** `rd = MEM[rs1 + offset][31:0]`

### LBU

- **Format:** I
- **Opcode:** `0x03`
- **Funct3:** `0x4`
- **Operands:** `rd, offset(rs1)`
- **Description:** Load byte (zero-extended)
- **Operation:** `rd = MEM[rs1 + offset][7:0]`

### LHU

- **Format:** I
- **Opcode:** `0x03`
- **Funct3:** `0x5`
- **Operands:** `rd, offset(rs1)`
- **Description:** Load halfword (zero-extended)
- **Operation:** `rd = MEM[rs1 + offset][15:0]`

### SB

- **Format:** S
- **Opcode:** `0x23`
- **Funct3:** `0x0`
- **Operands:** `rs2, offset(rs1)`
- **Description:** Store byte
- **Operation:** `MEM[rs1 + offset][7:0] = rs2[7:0]`

### SH

- **Format:** S
- **Opcode:** `0x23`
- **Funct3:** `0x1`
- **Operands:** `rs2, offset(rs1)`
- **Description:** Store halfword
- **Operation:** `MEM[rs1 + offset][15:0] = rs2[15:0]`

### SRAI

- **Format:** I
- **Opcode:** `0x13`
- **Funct3:** `0x5`
- **Operands:** `rd, rs1, shamt5`
- **Description:** Arithmetic right shift by immediate shift amount
- **Operation:** `rd = rs1 >>> shamt`

### SLLI

- **Format:** I
- **Opcode:** `0x13`
- **Funct3:** `0x1`
- **Operands:** `rd, rs1, shamt5`
- **Description:** Logical left shift by immediate shift amount
- **Operation:** `rd = rs1 << shamt`

### SRA

- **Format:** R
- **Opcode:** `0x33`
- **Funct3:** `0x5`
- **Funct7:** `0x20`
- **Operands:** `rd, rs1, rs2`
- **Description:** Arithmetic right shift by lower 5 bits of rs2
- **Operation:** `rd = rs1 >>> rs2[4:0]`

### SRL

- **Format:** R
- **Opcode:** `0x33`
- **Funct3:** `0x5`
- **Funct7:** `0x00`
- **Operands:** `rd, rs1, rs2`
- **Description:** Logical right shift by lower 5 bits of rs2
- **Operation:** `rd = rs1 >> rs2[4:0]`

### SLL

- **Format:** R
- **Opcode:** `0x33`
- **Funct3:** `0x1`
- **Funct7:** `0x00`
- **Operands:** `rd, rs1, rs2`
- **Description:** Logical left shift by lower 5 bits of rs2
- **Operation:** `rd = rs1 << rs2[4:0]`

### SRLI

- **Format:** I
- **Opcode:** `0x13`
- **Funct3:** `0x5`
- **Operands:** `rd, rs1, shamt5`
- **Description:** Logical right shift by immediate shift amount
- **Operation:** `rd = rs1 >> shamt`

### FENCEI

- **Format:** I
- **Opcode:** `0x0F`
- **Funct3:** `0x1`
- **Operands:** `—`
- **Description:** Instruction fence — synchronizes instruction and data streams
- **Operation:** `Flushes instruction cache after data writes`

### FENCE

- **Format:** I
- **Opcode:** `0x0F`
- **Funct3:** `0x0`
- **Operands:** `pred, succ`
- **Description:** Memory ordering fence
- **Operation:** `Orders memory accesses as specified by pred and succ fields`

### CSRRCI

- **Format:** I
- **Opcode:** `0x73`
- **Funct3:** `0x7`
- **Operands:** `rd, csr, uimm5`
- **Description:** Atomic read and clear bits in CSR with immediate
- **Operation:** `rd = CSR[csr]; CSR[csr] &= ~zimm`

### ECALL

- **Format:** I
- **Opcode:** `0x73`
- **Funct3:** `0x0`
- **Operands:** `—`
- **Description:** Environment call — raises a system call exception
- **Operation:** `Traps to the configured exception handler`

### EBREAK

- **Format:** I
- **Opcode:** `0x73`
- **Funct3:** `0x0`
- **Operands:** `—`
- **Description:** Breakpoint — raises a breakpoint exception
- **Operation:** `Used by debuggers to halt program execution`

### CSRRW

- **Format:** I
- **Opcode:** `0x73`
- **Funct3:** `0x1`
- **Operands:** `rd, csr, rs1`
- **Description:** Atomic read/write CSR — writes rs1 to CSR, returns old value in rd
- **Operation:** `rd = CSR[csr]; CSR[csr] = rs1`

### CSRRS

- **Format:** I
- **Opcode:** `0x73`
- **Funct3:** `0x2`
- **Operands:** `rd, csr, rs1`
- **Description:** Atomic read and set bits in CSR
- **Operation:** `rd = CSR[csr]; CSR[csr] |= rs1`

### CSRRC

- **Format:** I
- **Opcode:** `0x73`
- **Funct3:** `0x3`
- **Operands:** `rd, csr, rs1`
- **Description:** Atomic read and clear bits in CSR
- **Operation:** `rd = CSR[csr]; CSR[csr] &= ~rs1`

### CSRRWI

- **Format:** I
- **Opcode:** `0x73`
- **Funct3:** `0x5`
- **Operands:** `rd, csr, uimm5`
- **Description:** Atomic read/write CSR with immediate
- **Operation:** `rd = CSR[csr]; CSR[csr] = zimm`

### CSRRSI

- **Format:** I
- **Opcode:** `0x73`
- **Funct3:** `0x6`
- **Operands:** `rd, csr, uimm5`
- **Description:** Atomic read and set bits in CSR with immediate
- **Operation:** `rd = CSR[csr]; CSR[csr] |= zimm`

\newpage

# Opcode Map

The following table shows the top-level opcode mapping. The opcode occupies bits 6:0 of every instruction.

| opcode[6:0] | Type | Instructions |
|-------------|------|-------------|
| `0x33` (51) | R | ADD, SUB, SLT, SLTU, AND, OR, XOR, SLL, SRL, SRA (10 inst)s |
| `0x13` (19) | I | ADDI, SLTI, SLTIU, ANDI, ORI, XORI, SLLI, SRLI, SRAI (9 inst)s |
| `0x37` (55) | U | LUI (1 inst) |
| `0x17` (23) | U | AUIPC (1 inst) |
| `0x03` (3) | I | LB, LH, LW, LBU, LHU (5 inst)s |
| `0x23` (35) | S | SB, SH, SW (3 inst)s |
| `0x63` (99) | B | BEQ, BNE, BLT, BGE, BLTU, BGEU (6 inst)s |
| `0x6F` (111) | J | JAL (1 inst) |
| `0x67` (103) | I | JALR (1 inst) |
| `0x0F` (15) | I | FENCE, FENCEI (2 inst)s |
| `0x73` (115) | I | ECALL, EBREAK, CSRRW, CSRRS, CSRRC, CSRRWI, CSRRSI, CSRRCI (8 inst)s |

### Encoding Matrix

| opcode \ funct3 | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|---|
| `0x33` | ADD | SLL | SLT | SLTU | XOR | SRL | OR | AND |
| `0x13` | ADDI | SLLI | SLTI | SLTIU | XORI | SRLI | ORI | ANDI |
| `0x37` | — | — | — | — | — | — | — | — |
| `0x17` | — | — | — | — | — | — | — | — |
| `0x03` | LB | LH | LW | — | LBU | LHU | — | — |
| `0x23` | SB | SH | SW | — | — | — | — | — |
| `0x63` | BEQ | BNE | — | — | BLT | BGE | BLTU | BGEU |
| `0x6F` | — | — | — | — | — | — | — | — |
| `0x67` | JALR | — | — | — | — | — | — | — |
| `0x0F` | FENCE | FENCEI | — | — | — | — | — | — |
| `0x73` | ECALL | CSRRW | CSRRS | CSRRC | — | CSRRWI | CSRRSI | CSRRCI |

\newpage

# Exception and Trap Handling

## Exception Types

| Exception Code | Name | Description |
|----------------|------|-------------|
| 0 | Instruction address misaligned | PC not aligned to 4 bytes |
| 1 | Instruction access fault | Failed to fetch instruction |
| 2 | Illegal instruction | Unrecognized opcode |
| 3 | Breakpoint | EBREAK instruction executed |
| 4 | Load address misaligned | Load address not properly aligned |
| 5 | Load access fault | Failed to load from memory |
| 6 | Store address misaligned | Store address not properly aligned |
| 7 | Store access fault | Failed to store to memory |
| 8 | Environment call from U-mode | ECALL in user mode |
| 9 | Environment call from S-mode | ECALL in supervisor mode |
| 11 | Environment call from M-mode | ECALL in machine mode |

## Trap Vector

On taking a trap, the implementation:

1. Sets `mepc` to the PC of the trapping instruction (or next PC for ECALL/EBREAK)
2. Sets `mcause` to the exception code
3. Sets `mtval` to exception-specific information
4. Sets `mstatus.MPP` to the current privilege mode
5. Sets `mstatus.MPIE` to `mstatus.MIE`
6. Clears `mstatus.MIE`
7. Sets PC to `mtvec`

\newpage

# Pseudo-Instructions

| Pseudo-instruction | Expansion | Description |
|-------------------|-----------|-------------|
| `NOP` | `ADDI x0, x0, 0` | No operation |
| `MV rd, rs` | `ADDI rd, rs, 0` | Copy register |
| `NOT rd, rs` | `XORI rd, rs, -1` | Bitwise NOT |
| `NEG rd, rs` | `SUB rd, x0, rs` | Negate register |
| `LI rd, imm` | (multiple) | Load immediate |
| `LA rd, label` | (multiple) | Load address |
| `RET` | `JALR x0, x1, 0` | Return from subroutine |
| `CALL label` | (multiple) | Call subroutine |
| `J label` | `JAL x0, label` | Unconditional jump |
| `JR rs` | `JALR x0, rs, 0` | Jump register |

\newpage

# Control and Status Registers

The following CSRs are accessible via the `CSRRW`, `CSRRS`, `CSRRC`, and their immediate variants.

| Address | Name | Description |
|---------|------|-------------|
| `0x300` | mstatus | Machine status register — holds global interrupt enable and privilege state |
| `0x301` | misa | Machine ISA register — encodes supported ISA extensions |
| `0x302` | medeleg | Machine exception delegation register |
| `0x303` | mideleg | Machine interrupt delegation register |
| `0x304` | mie | Machine interrupt-enable register |
| `0x305` | mtvec | Machine trap-handler base address |
| `0x306` | mcounteren | Machine counter enable register |
| `0x340` | mscratch | Scratch register for machine-mode trap handlers |
| `0x341` | mepc | Machine exception program counter — holds PC of trapping instruction |
| `0x342` | mcause | Machine trap cause — encodes exception or interrupt cause |
| `0x343` | mtval | Machine trap value — exception-specific information |
| `0x344` | mip | Machine interrupt pending register |
| `0x3A0` | pmpcfg0 | Physical memory protection configuration 0 |
| `0x3B0` | pmpaddr0 | Physical memory protection address 0 |
| `0x3B1` | pmpaddr1 | Physical memory protection address 1 |
| `0x3B2` | pmpaddr2 | Physical memory protection address 2 |
| `0x3B3` | pmpaddr3 | Physical memory protection address 3 |
| `0xB00` | mcycle | Machine cycle counter — counts number of clock cycles |
| `0xB02` | minstret | Machine instructions-retired counter |
| `0xB03` | mhpmcounter3 | Machine hardware performance counter 3 |
| `0xB04` | mhpmcounter4 | Machine hardware performance counter 4 |
| `0xB05` | mhpmcounter5 | Machine hardware performance counter 5 |
| `0x323` | mhpmevent3 | Machine hardware performance event selector 3 |
| `0x324` | mhpmevent4 | Machine hardware performance event selector 4 |
| `0x325` | mhpmevent5 | Machine hardware performance event selector 5 |
| `0xC00` | ucycle | User-mode cycle counter read |
| `0xC02` | uinstret | User-mode instructions-retired read |
| `0xC80` | ucycleh | Upper 32 bits of user-mode cycle counter |
| `0xC82` | uinstreth | Upper 32 bits of user-mode instructions-retired counter |
| `0x100` | sstatus | Supervisor status register |
| `0x104` | sie | Supervisor interrupt-enable register |
| `0x105` | stvec | Supervisor trap-handler base address |
| `0x140` | sscratch | Scratch register for supervisor-mode trap handlers |
| `0x141` | sepc | Supervisor exception program counter |
| `0x142` | scause | Supervisor trap cause register |
| `0x143` | stval | Supervisor trap value register |
| `0x144` | sip | Supervisor interrupt pending register |
| `0x180` | satp | Supervisor address translation and protection — controls page tables |

\newpage

# Quick Reference

## Instruction Encoding Quick Reference

### R-Type

```
 31:27   | 26:25  | 24:20  | 19:15  | 14:12  | 11:7   | 6:0
 funct7 |  —     | rs2    | rs1    | funct3 | rd     | opcode
   7    |  2     |   5    |   5    |   3    |   5    |   7
```

### I-Type

```
 31:20   | 19:15  | 14:12  | 11:7   | 6:0
 imm[11:0]| rs1    | funct3 | rd     | opcode
   12    |   5    |   3    |   5    |   7
```

### S-Type

```
 31:27  | 26:25  | 24:20  | 19:15  | 14:12  | 11:7   | 6:0
 imm[11:5]| —     | rs2    | rs1    | funct3 | imm[4:0]| opcode
   7    |  2     |   5    |   5    |   3    |   5    |   7
```

### B-Type

```
 31    | 30:27 | 26:25 | 24:20  | 19:15  | 14:12  | 11:8  | 7    | 6:0
 imm[12]| imm[10:5]| —  | rs2    | rs1    | funct3 | imm[4:1]| imm[11]| opcode
   1   |   6   |  2    |   5    |   5    |   3    |   4   |  1   |   7
```

### U-Type

```
 31:12      | 11:7   | 6:0
 imm[31:12] | rd     | opcode
   20       |   5    |   7
```

### J-Type

```
 31    | 30:21  | 20   | 19:12     | 11:7   | 6:0
 imm[20]| imm[10:1]| imm[11]| imm[19:12]| rd     | opcode
   1   |   10   |  1   |   8       |   5    |   7
```