diff --git a/tools/emul/bshell/shell.c b/tools/emul/bshell/shell.c
index 31df626..2560414 100644
--- a/tools/emul/bshell/shell.c
+++ b/tools/emul/bshell/shell.c
@@ -186,5 +186,6 @@ int main()
     termInfo.c_lflag |= ECHO;
     termInfo.c_lflag |= ICANON;
     tcsetattr(0, TCSAFLUSH, &termInfo);
+    emul_printdebug();
     return 0;
 }
diff --git a/tools/emul/emul.c b/tools/emul/emul.c
index d060123..e39dd07 100644
--- a/tools/emul/emul.c
+++ b/tools/emul/emul.c
@@ -48,6 +48,7 @@ Machine* emul_init()
 {
     memset(m.mem, 0, 0x10000);
     m.ramstart = 0;
+    m.minsp = 0xffff;
     for (int i=0; i<0x100; i++) {
         m.iord[i] = NULL;
         m.iowr[i] = NULL;
@@ -65,6 +66,10 @@ bool emul_step()
 {
     if (!m.cpu.halted) {
         Z80Execute(&m.cpu);
+        ushort newsp = m.cpu.R1.wr.SP;
+        if (newsp != 0 && newsp < m.minsp) {
+            m.minsp = newsp;
+        }
         return true;
     } else {
         return false;
@@ -75,3 +80,8 @@ void emul_loop()
 {
     while (emul_step());
 }
+
+void emul_printdebug()
+{
+    fprintf(stderr, "Min SP: %04x\n", m.minsp);
+}
diff --git a/tools/emul/emul.h b/tools/emul/emul.h
index 61d2b8f..8b47400 100644
--- a/tools/emul/emul.h
+++ b/tools/emul/emul.h
@@ -2,15 +2,17 @@
 #include <stdbool.h>
 #include "libz80/z80.h"
 
-typedef uint8_t (*IORD) ();
-typedef void (*IOWR) (uint8_t data);
+typedef byte (*IORD) ();
+typedef void (*IOWR) (byte data);
 
 typedef struct {
     Z80Context cpu;
-    uint8_t mem[0x10000];
+    byte mem[0x10000];
     // Set to non-zero to specify where ROM ends. Any memory write attempt
     // below ramstart will trigger a warning.
-    uint16_t ramstart;
+    ushort ramstart;
+    // The minimum value reached by SP at any point during execution.
+    ushort minsp;
     // Array of 0x100 function pointers to IO read and write routines. Leave to
     // NULL when IO port is unhandled.
     IORD iord[0x100];
@@ -20,3 +22,4 @@ typedef struct {
 Machine* emul_init();
 bool emul_step();
 void emul_loop();
+void emul_printdebug();
diff --git a/tools/emul/shell/shell.c b/tools/emul/shell/shell.c
index cb74f2a..215dace 100644
--- a/tools/emul/shell/shell.c
+++ b/tools/emul/shell/shell.c
@@ -186,5 +186,6 @@ int main()
     termInfo.c_lflag |= ECHO;
     termInfo.c_lflag |= ICANON;
     tcsetattr(0, TCSAFLUSH, &termInfo);
+    emul_printdebug();
     return 0;
 }