From 315a07cbc76ce5b515b7256265a6fc32bd51652c Mon Sep 17 00:00:00 2001
From: Mark Pizzolato <mark@infocomm.com>
Date: Sun, 13 Feb 2022 22:50:37 -0800
Subject: [PATCH] SCP: Avoid writing out memory buffered devices on detach that
 haven't changed

As discussed in #1109
---
 scp.c      | 22 +++++++++++++++++-----
 sim_defs.h |  1 +
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/scp.c b/scp.c
index 181ab7fe..df835885 100644
--- a/scp.c
+++ b/scp.c
@@ -8143,13 +8143,22 @@ else {
     }
 if (uptr->flags & UNIT_BUFABLE) {                       /* buffer? */
     uint32 cap = ((uint32) uptr->capac) / dptr->aincr;  /* effective size */
-    if (uptr->flags & UNIT_MUSTBUF)                     /* dyn alloc? */
+    if (uptr->flags & UNIT_MUSTBUF) {                   /* dyn alloc? */
         uptr->filebuf = calloc (cap, SZ_D (dptr));      /* allocate */
-    if (uptr->filebuf == NULL)                          /* no buffer? */
-        return attach_err (uptr, SCPE_MEM);             /* error */
+        uptr->filebuf2 = calloc (cap, SZ_D (dptr));     /* allocate copy */
+        if ((uptr->filebuf == NULL) ||                  /* no buffer? */
+            (uptr->filebuf2 == NULL)) {
+            free (uptr->filebuf);
+            uptr->filebuf = NULL;
+            free (uptr->filebuf2);
+            uptr->filebuf2 = NULL;
+            return attach_err (uptr, SCPE_MEM);             /* error */
+            }
+        }
     sim_messagef (SCPE_OK, "%s: buffering file in memory\n", sim_uname (uptr));
     uptr->hwmark = (uint32)sim_fread (uptr->filebuf,    /* read file */
         SZ_D (dptr), cap, uptr->fileref);
+    memcpy (uptr->filebuf2, uptr->filebuf, cap * SZ_D (dptr));/* save initial contents */
     uptr->flags = uptr->flags | UNIT_BUF;               /* set buffered */
     }
 uptr->flags = uptr->flags | UNIT_ATT;
@@ -8268,7 +8277,8 @@ if ((dptr = find_dev_from_unit (uptr)) == NULL)
     return SCPE_OK;
 if ((uptr->flags & UNIT_BUF) && (uptr->filebuf)) {
     uint32 cap = (uptr->hwmark + dptr->aincr - 1) / dptr->aincr;
-    if (uptr->hwmark && ((uptr->flags & UNIT_RO) == 0)) {
+    if (((uptr->flags & UNIT_RO) == 0) && 
+        (memcmp (uptr->filebuf, uptr->filebuf2, (size_t)(SZ_D (dptr) * (uptr->capac / dptr->aincr))) != 0)) {
         sim_messagef (SCPE_OK, "%s: writing buffer to file: %s\n", sim_uname (uptr), uptr->filename);
         rewind (uptr->fileref);
         sim_fwrite (uptr->filebuf, SZ_D (dptr), cap, uptr->fileref);
@@ -8276,8 +8286,10 @@ if ((uptr->flags & UNIT_BUF) && (uptr->filebuf)) {
             sim_printf ("%s: I/O error - %s", sim_uname (uptr), strerror (errno));
         }
     if (uptr->flags & UNIT_MUSTBUF) {                   /* dyn alloc? */
-        free (uptr->filebuf);                           /* free buf */
+        free (uptr->filebuf);                           /* free buffers */
         uptr->filebuf = NULL;
+        free (uptr->filebuf2);
+        uptr->filebuf2 = NULL;
         }
     uptr->flags = uptr->flags & ~UNIT_BUF;
     }
diff --git a/sim_defs.h b/sim_defs.h
index 4b5ceb15..9bbb99e4 100644
--- a/sim_defs.h
+++ b/sim_defs.h
@@ -585,6 +585,7 @@ struct UNIT {
     uint32              dynflags;                       /* dynamic flags */
     t_addr              capac;                          /* capacity */
     t_addr              pos;                            /* file position */
+    void                *filebuf2;                      /* copy of initial memory buffer */
     void                (*io_flush)(UNIT *up);          /* io flush routine */
     uint32              iostarttime;                    /* I/O start time */
     int32               buf;                            /* buffer */