From 797a3087647aa2a377cf3ef7c776d0c7dd4dafbb Mon Sep 17 00:00:00 2001
From: Konstantin Nazarov <mail@knazarov.com>
Date: Fri, 6 Dec 2024 23:50:21 +0000
Subject: [PATCH] Add a simple boot section

---
 example/Makefile  | 6 ++++--
 example/boot.s    | 6 ++++++
 example/linker.ld | 2 +-
 src/vm.cpp        | 7 ++++++-
 4 files changed, 17 insertions(+), 4 deletions(-)
 create mode 100644 example/boot.s

diff --git a/example/Makefile b/example/Makefile
index 1f98c74..372e116 100644
--- a/example/Makefile
+++ b/example/Makefile
@@ -1,4 +1,6 @@
-example: example.c
-	riscv32-none-elf-gcc -fno-builtin -fvisibility=hidden -nostdlib -nostartfiles -march=rv32im -mabi=ilp32 -o example example.c -T linker.ld
+example: example.c Makefile boot.s linker.ld
+	riscv32-none-elf-as -march=rv32i -mabi=ilp32 boot.s -o boot.o
+	riscv32-none-elf-gcc -fno-builtin -fvisibility=hidden -nostdlib -nostartfiles -march=rv32im -mabi=ilp32 -c example.c -o example.o
+	riscv32-none-elf-ld boot.o example.o -T linker.ld -o example
 	riscv32-none-elf-strip -R .riscv.attributes example
 	riscv32-none-elf-strip -R .comment example
diff --git a/example/boot.s b/example/boot.s
new file mode 100644
index 0000000..35fd4a2
--- /dev/null
+++ b/example/boot.s
@@ -0,0 +1,6 @@
+.globl _boot
+_boot:
+    lui x2, 0x80004
+    jal main
+    sbreak
+    j .
diff --git a/example/linker.ld b/example/linker.ld
index a5ea337..bb9558b 100644
--- a/example/linker.ld
+++ b/example/linker.ld
@@ -1,7 +1,7 @@
 ENTRY(_start)
 
 SECTIONS {
-    . = 0x1000;  /* Start address of the program */
+    . = 0x0000;  /* Start address of the program */
 
     .text : {
         *(.text)   /* Place all .text sections (code) here */
diff --git a/src/vm.cpp b/src/vm.cpp
index 73f3505..c7f2098 100644
--- a/src/vm.cpp
+++ b/src/vm.cpp
@@ -23,7 +23,8 @@ void eval(uint8_t* memory, size_t memory_size) {
     return instruction;
   };
 
-  while (pc < memory_size) {
+  bool running = true;
+  while (pc < memory_size && running) {
     uint32_t instr = fetch_instruction();
     if (instr == 0) break;
     // std::cout << "pc: " << pc << "\n";
@@ -248,6 +249,10 @@ void eval(uint8_t* memory, size_t memory_size) {
       registers[rd] = pc + (imm << 12);  // Add the immediate (shifted left) to the current PC
       break;
     }
+    case 0x73: { // EBREAK
+      running = false; // Flag to stop the emulator
+      break;
+    }
     default:
       throw std::runtime_error("Unknown opcode");
     }