Implement 0BRANCH

diff --git a/main.asm b/main.asm
index e5444b87a58ba20ad0f984627d05ef0e99efecec..af84a66631a0d17d5abb5922a220130f9135f8b4 100644 (file)
--- a/main.asm
+++ b/main.asm
@@ -59,6 +59,23 @@ LIT:
   push rax
   next
 
+;; 0BRANCH is the fundamental mechanism for branching. If the top of the stack
+;; is zero, we jump by the given offset. 0BRANCH is given the offset as an
+;; integer after the word.
+ZBRANCH:
+  dq .start
+.start:
+  ;; Compare top of stack to see if we should branch
+  pop rax
+  cmp rax, 0
+  jnz .dont_branch
+.do_branch:
+  add rsi, [rsi] ; [RSI], which is the next word, contains the offset; we add this to the instruction pointer.
+  next           ; Then, we can just continue execution as normal
+.dont_branch:
+  add rsi, 8     ; We need to skip over the next word, which contains the offset.
+  next
+
 ;; Expects a character on the stack and prints it to standard output.
 EMIT:
   dq .start