implement branch and 0branch; fix jmp_cc_rel_imm8

Force-Push: yes
Change-Id: I45c0a5e729cae2493e341ad3b6145708826383ab

1 files changed, 19 insertions, 5 deletions

diff --git a/quine.asm b/quine.asm
index c92f1cd..4eb5017 100644
--- a/quine.asm
+++ b/quine.asm
@@ -2328,14 +2328,28 @@ cold_start:
   dq pack_next, lit, 8, packalign, early_here_store
 
   dq litstring, "branch", early_create, early_self_codeword, early_here, fetch
-  ; TODO
-  dq lit, 8, packalign, early_here_store
+  ; Save the address we're packing to; it will be the label we jump to from
+  ; zbranch.
+  dq dup, unroll3
+  dq rsi, rsi, add_reg64_indirect_reg64
+  dq pack_next, lit, 8, packalign, early_here_store
 
   ; This was "zbranch".
   dq litstring, "0branch", early_create, early_self_codeword, early_here
   dq fetch
-  ; TODO
-  dq lit, 8, packalign, early_here_store
+  dq rax, pop_reg64
+  dq rax, rax, test_reg64_reg64
+  ; Retrieve the saved address for the start of "branch". Compute the
+  ; relative offset from the current address to it. Add two bytes to allow
+  ; for the length of the jmp instruction.
+  dq dup, lit, 4, roll, sub, lit, 2, add
+  ; It's slightly counterintuitive that the condition is called cc_equal;
+  ; that's a result of the condition names favoring cmp over test. While cmp
+  ; simulates subtraction, test simulates biwise AND. What we're testing is
+  ; that the result is zero.
+  dq cc_equal, jmp_cc_rel_imm8
+  dq lods64
+  dq pack_next, lit, 8, packalign, early_here_store
 
   ; This name is exactly eight bytes long. Don't even ask (go read litstring's
   ; code if you really need to know).
@@ -4249,7 +4263,7 @@ defword set_reg8_cc, 0
 ;   condition code name
 defword jmp_cc_rel_imm8, 0
   dq docol
-  dq unroll3, swap, conditioncode, lit, 0x70, opcodecc
+  dq roll3, swap, conditioncode, lit, 0x70, opcodecc
   dq swap, pack8
   dq exit