From 938b75fc446e4e9ef6e07f98793902956295b77e Mon Sep 17 00:00:00 2001
From: Mark Pizzolato <mark@infocomm.com>
Date: Thu, 22 Aug 2013 15:50:08 -0700
Subject: [PATCH] Clear storage and branch preserves B register (Van Snyder)

---
 I1401/i1401_cpu.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/I1401/i1401_cpu.c b/I1401/i1401_cpu.c
index 61aaf155..dcdce7be 100644
--- a/I1401/i1401_cpu.c
+++ b/I1401/i1401_cpu.c
@@ -1,6 +1,6 @@
 /* i1401_cpu.c: IBM 1401 CPU simulator
 
-   Copyright (c) 1993-2011, Robert M. Supnik
+   Copyright (c) 1993-2012, Robert M. Supnik
 
    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
@@ -23,6 +23,7 @@
    used in advertising or otherwise to promote the sale, use or other dealings
    in this Software without prior written authorization from Robert M Supnik.
 
+   08-Oct-12    RMS     Clear storage and branch preserves B register (Van Snyder)
    19-Mar-11    RMS     Reverted multiple tape indicator implementation
    20-Jan-11    RMS     Fixed branch on EOT indicator per hardware (Van Snyder)
    07-Nov-10    RMS     Fixed divide not to clear word marks in quotient
@@ -178,6 +179,14 @@ typedef struct {
                         PCQ_ENTRY; \
                         IS = AS;
 
+#define BRANCH_CS       if (ADDR_ERR (AS)) { \
+                            reason = STOP_INVBR; \
+                            break; \
+                            } \
+                        PCQ_ENTRY; \
+                        IS = AS;
+
+
 uint8 M[MAXMEMSIZE] = { 0 };                            /* main memory */
 int32 saved_IS = 0;                                     /* saved IS */
 int32 AS = 0;                                           /* AS */
@@ -1563,6 +1572,9 @@ CHECK_LENGTH:
    5,6          invalid B-address
    7            branch
    8+           one operand, branch ignored
+
+   Note that clear storage and branch does not overwrite the B register,
+   unlike all other branches
 */
 
     case OP_CS:                                         /* clear storage */
@@ -1572,7 +1584,7 @@ CHECK_LENGTH:
         if (BS < 0)                                     /* wrap if needed */
             BS = BS + MEMSIZE;
         if (ilnt == 7) {                                /* branch variant? */
-            BRANCH;
+            BRANCH_CS;                                  /* special branch */
             }
         break;