Make push opcodes a specific bit set and add a NOOP opcode

By default, a zero initialised set of instructions are NOOPs, which is
great.

Last two bits of a push opcode is always 01.  Rest of the bits are
used to distinguish between differing types of push.  This makes it
easier to inspect on the byte level what type of opcode we have.

2 files changed, 14 insertions, 9 deletions

diff --git a/src/inst.h b/src/inst.h
index 95baaf2..086696a 100644
--- a/src/inst.h
+++ b/src/inst.h
@@ -17,14 +17,20 @@
 
 typedef enum
 {
-  OP_PUSH_BYTE = 1,
-  OP_PUSH_WORD,
-  OP_PUSH_FLOAT,
-} op_t;
+  OP_NOOP = 0,
+
+  OP_PUSH_BYTE  = 0b0001,
+  OP_PUSH_WORD  = 0b0101,
+  OP_PUSH_FLOAT = 0b1001,
+
+  OP_HALT,
+} opcode_t;
+
+#define OPCODE_IS_PUSH(OPCODE) (((OPCODE)&1) == 1)
 
 typedef struct
 {
-  op_t opcode;
+  opcode_t opcode;
   data_t operand;
 } inst_t;
 
diff --git a/src/main.c b/src/main.c
index 9b744b2..4ccb42a 100644
--- a/src/main.c
+++ b/src/main.c
@@ -121,11 +121,10 @@ void vm_execute(vm_t *vm)
     // TODO: Error (Went past end of program)
     return;
   inst_t instruction = prog->instructions[prog->ptr];
-  // NOTE: This is ballsy; I'm essentially saying I will never use the
-  // last 2 bits unless it's a push routine
-  if ((instruction.opcode & 0b11) != 0)
+
+  // Check if opcode is PUSH_LIKE
+  if (OPCODE_IS_PUSH(instruction.opcode))
   {
-    // Possible push routines
     typedef void (*push_f)(vm_t *, data_t);
     const push_f routines[] = {[OP_PUSH_BYTE]  = vm_push_byte,
                                [OP_PUSH_WORD]  = vm_push_word,