Implement DB4 instruction for unit tests

When the DB4 instruction is evaluated, a single '.' character will be
printed to stdout if the return stack is empty and the working stack
contains a single 0xff byte, otherwise a single 'X' character will be
printed to stdout.

This is to facilitate unit testing the instruction set with a Bedrock
program, from within the emulator.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/emulator.rs b/src/emulator.rs
index 4909a27..d983378 100644
--- a/src/emulator.rs
+++ b/src/emulator.rs
@@ -93,6 +93,15 @@ impl BedrockEmulator {
             DebugVariant::DB3 => {
                 // Only resets the debug timer
             }
+            DebugVariant::DB4 => {
+                if self.vm.wst.sp == 1
+                && self.vm.rst.sp == 0
+                && self.vm.wst.mem[0] == 0xff {
+                    print!(".")
+                } else {
+                    print!("X")
+                }
+            }
             _ => (),
         }
         self.cycles_elapsed = self.vm.cycles;