From 804da393a82713b8f088be3e16460208be35ecfe Mon Sep 17 00:00:00 2001
From: Dominic Grimm <dominic@dergrimm.net>
Date: Fri, 31 Mar 2023 14:36:00 +0200
Subject: [PATCH] Make 'pop' operation safe

---
 examples/testforth.asm | 3 +++
 examples/testforth.fth | 2 ++
 hence/src/emulator.rs  | 4 ++++
 3 files changed, 9 insertions(+)

diff --git a/examples/testforth.asm b/examples/testforth.asm
index 1c5491f..8a25fa2 100644
--- a/examples/testforth.asm
+++ b/examples/testforth.asm
@@ -131,4 +131,7 @@ return_call_stack_jump:
   .std_alu CORE_ALU_GT
   tlr CORE_REG_A
   .std_cond_jump loop_strings_1
+  dbg
+  pop
+  dbg
 .std_stop
diff --git a/examples/testforth.fth b/examples/testforth.fth
index 2694a60..4f7c413 100644
--- a/examples/testforth.fth
+++ b/examples/testforth.fth
@@ -2,3 +2,5 @@
 
 ." test\n"
 ." jdsafhjfnjfn\n"
+
+debug drop debug
diff --git a/hence/src/emulator.rs b/hence/src/emulator.rs
index aaa58e1..64ed074 100644
--- a/hence/src/emulator.rs
+++ b/hence/src/emulator.rs
@@ -247,7 +247,11 @@ impl Emulator {
                 self.reg_sp = self.reg_sp.wrapping_add(1);
             }
             0x02 => {
+                let before = self.reg_sp;
                 self.reg_sp = self.reg_sp.wrapping_sub(1);
+                if before < self.reg_sp {
+                    self.reg_sp = 0;
+                }
             }
             0x03 => {
                 self.tmp = self.reg_arg;