cannon: use constant instead of magic value

cannon/mipsevm/exec/calling_convention.go

@@ -0,0 +1,27 @@
+package exec
+
+// FYI: https://en.wikibooks.org/wiki/MIPS_Assembly/Register_File
+//
+//	https://refspecs.linuxfoundation.org/elf/mipsabi.pdf
+const (
+	// syscall number; 1st return value
+	RegV0 = 2
+	// syscall arguments; returned unmodified
+	RegA0 = 4
+	RegA1 = 5
+	RegA2 = 6
+	// 4th syscall argument; set to 0/1 for success/error
+	RegA3 = 7
+)
+
+// FYI: https://web.archive.org/web/20231223163047/https://www.linux-mips.org/wiki/Syscall
+
+const (
+	RegSyscallNum    = RegV0
+	RegSyscallErrno  = RegA3
+	RegSyscallRet1   = RegV0
+	RegSyscallParam1 = RegA0
+	RegSyscallParam2 = RegA1
+	RegSyscallParam3 = RegA2
+	RegSyscallParam4 = RegA3
+)

cannon/mipsevm/exec/mips_syscalls.go

@@ -99,12 +99,12 @@ const (
 )
 
 func GetSyscallArgs(registers *[32]Word) (syscallNum, a0, a1, a2, a3 Word) {
-	syscallNum = registers[2] // v0
+	syscallNum = registers[RegSyscallNum] // v0
 
-	a0 = registers[4]
-	a1 = registers[5]
-	a2 = registers[6]
-	a3 = registers[7]
+	a0 = registers[RegSyscallParam1]
+	a1 = registers[RegSyscallParam2]
+	a2 = registers[RegSyscallParam3]
+	a3 = registers[RegSyscallParam4]
 
 	return syscallNum, a0, a1, a2, a3
 }
@@ -281,8 +281,8 @@ func HandleSysFcntl(a0, a1 Word) (v0, v1 Word) {
 }
 
 func HandleSyscallUpdates(cpu *mipsevm.CpuScalars, registers *[32]Word, v0, v1 Word) {
-	registers[2] = v0
-	registers[7] = v1
+	registers[RegSyscallRet1] = v0
+	registers[RegSyscallErrno] = v1
 
 	cpu.PC = cpu.NextPC
 	cpu.NextPC = cpu.NextPC + 4

cannon/mipsevm/multithreaded/mips.go

@@ -59,8 +59,8 @@ func (m *InstrumentedState) handleSyscall() error {
 
 		newThread.Registers[29] = a1
 		// the child will perceive a 0 value as returned value instead, and no error
-		newThread.Registers[2] = 0
-		newThread.Registers[7] = 0
+		newThread.Registers[exec.RegSyscallRet1] = 0
+		newThread.Registers[exec.RegSyscallErrno] = 0
 		m.state.NextThreadId++
 
 		// Preempt this thread for the new one. But not before updating PCs