From 8024af6b1348b5f47fabe5a1949de54607a33888 Mon Sep 17 00:00:00 2001
From: taitep <taitep@taitep.se>
Date: Sat, 27 Dec 2025 21:47:22 +0100
Subject: [PATCH] Implement ECALL and EBREAK, the final RV64I instructions!

---
 README.md           |  2 ++
 src/decode.rs       |  5 +++++
 src/exceptions.rs   |  9 +++++++++
 src/instructions.rs | 14 +++++++++++++-
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index de9ca7f..697c0f2 100644
--- a/README.md
+++ b/README.md
@@ -4,6 +4,8 @@ taitep's RISC-V Emulator.
 The goal is to support at least RV64GC and be able to run Linux,
 potentially more. No plans for RV32I or RV32/64E.
 
+Currently implemented RISC-V ISA: `RV64I`
+
 ## Current Use
 Currently, the emulator is nowhere near complete,
 its not even at rv64i, but it does work for a subset of it.
diff --git a/src/decode.rs b/src/decode.rs
index d520151..e24bac0 100644
--- a/src/decode.rs
+++ b/src/decode.rs
@@ -93,4 +93,9 @@ impl Instruction {
     pub fn funct6(self) -> u8 {
         (self.0 >> 26 & 0x3f) as u8
     }
+
+    /// Mostly/only used for the SYSTEM opcode
+    pub fn funct12(self) -> u16 {
+        (self.0 >> 20) as u16
+    }
 }
diff --git a/src/exceptions.rs b/src/exceptions.rs
index 5e3f02f..6bc21f2 100644
--- a/src/exceptions.rs
+++ b/src/exceptions.rs
@@ -31,6 +31,15 @@ pub enum ExceptionType {
     HardwareError = 19,
 }
 
+impl ExceptionType {
+    pub fn with_no_value(self) -> Exception {
+        Exception {
+            type_: self,
+            value: 0,
+        }
+    }
+}
+
 #[derive(Debug, Clone, Copy)]
 pub struct Exception {
     pub type_: ExceptionType,
diff --git a/src/instructions.rs b/src/instructions.rs
index fe79684..99df830 100644
--- a/src/instructions.rs
+++ b/src/instructions.rs
@@ -13,7 +13,10 @@ use crate::{
     consts::DWord,
     core::Core,
     decode::Instruction,
-    exceptions::{Exception, ExceptionType::IllegalInstruction},
+    exceptions::{
+        Exception,
+        ExceptionType::{self, IllegalInstruction},
+    },
 };
 
 fn illegal(instr: Instruction) -> Result<(), Exception> {
@@ -130,6 +133,15 @@ pub(crate) fn find_and_exec(instr: Instruction, core: &mut Core) -> Result<(), E
             }
             _ => illegal(instr),
         },
+        0b11100 => match (instr.funct3(), instr.funct12(), instr.rs1(), instr.rd()) {
+            (0b000, 0b000000000000, 0, 0) => {
+                // TODO: When privilege modes are added, make the exception raised by ecall
+                // depend on privilege mode
+                Err(ExceptionType::EnvironmentCallFromMMode.with_no_value())
+            }
+            (0b000, 0b000000000001, 0, 0) => Err(ExceptionType::Breakpoint.with_no_value()),
+            _ => illegal(instr),
+        },
         _ => illegal(instr),
     }
 }