feat(sim): basic syscall framework

Author: EmbroiderSnow

sim/include/cpu.h

@@ -3,9 +3,15 @@
 
 #include <stdint.h>
 
+#define CSR_MEPC 0x341
+#define CSR_MCAUSE 0x342
+#define CSR_MSTATUS 0x300
+#define CSR_MTVEC 0x305
+
 typedef struct {
     uint64_t reg[32];
     uint64_t pc;
+    uint64_t csr[4096];
 } CPU_state;
 
 void init_cpu();

sim/include/trap.h

@@ -0,0 +1,11 @@
+#ifndef SYSCALL_H
+#define SYSCALL_H
+
+#include <decode.h>
+
+#define SYSCALL_EXIT 93
+#define SYSCALL_WRITE 64
+
+void handle_syscall(Decode *s);
+
+#endif

sim/src/cpu.c

@@ -8,6 +8,7 @@ static int running = 1;
 void init_cpu(){
     cpu.pc = MEM_BASE;
     memset(cpu.reg, 0, sizeof(cpu.reg));
+    memset(cpu.csr, 0, sizeof(cpu.csr));
 }
 
 

sim/src/decode.c

@@ -4,6 +4,7 @@
 #include <cpu.h>
 #include <memory.h>
 #include <decode.h>
+#include <trap.h>
 
 extern CPU_state cpu;
 
@@ -45,6 +46,13 @@ static void decode_operand(Decode *s, int *rd, uint64_t *src1, uint64_t *src2, u
         case TYPE_J:                   immJ(); break;
     }
 }
+
+static void ecall_handler(Decode *s) {
+    cpu.csr[CSR_MEPC] = s->snpc;
+    cpu.csr[CSR_MCAUSE] = 8;
+    handle_syscall(s);
+}
+
 void decode_exec(Decode *s){
     int rd = 0;
     uint64_t src1 = 0, src2 = 0, imm = 0;
@@ -121,6 +129,7 @@ void decode_exec(Decode *s){
 
     // Related to System Calls
     // ECALL
+    INSTPAT("0000000 00000 00000 000 00000 11100 11", ecall  , N, ecall_handler(s));
     // CSRRW, CSRRS, CSRRC, CSRRWI, CSRRSI, CSRRCI
 
     // RV64M

sim/src/syscall.c

@@ -0,0 +1,37 @@
+#include <cpu.h>
+#include <trap.h>
+#include <decode.h>
+
+extern CPU_state cpu;
+
+void handle_syscall(Decode *s) {
+    uint64_t syscall_no = cpu.reg[17]; // a7 register
+    switch (syscall_no) {
+        case SYSCALL_EXIT:
+            Log("Syscall: exit with code %lu", cpu.reg[10]); // a0 register
+            halt_trap(s->pc, cpu.reg[10]);
+            break;
+        
+        case SYSCALL_WRITE:
+            {
+                uint64_t fd = cpu.reg[10]; // a0
+                uint64_t buf = cpu.reg[11]; // a1
+                uint64_t count = cpu.reg[12]; // a2
+                if (fd == 1) { // stdout
+                    for (uint64_t i = 0; i < count; i++) {
+                        putchar(*(uint8_t *)(buf + i));
+                    }
+                    cpu.reg[10] = count; // return value in a0
+                } else {
+                    Log("Syscall: write to unsupported fd %lu", fd);
+                    cpu.reg[10] = -1; // error
+                }
+            }
+            break;
+        
+        default:
+            Log("Unknown syscall: %lu", syscall_no);
+            halt_trap(s->pc, -1);
+            break;
+    }
+}