From 81acc54ca0b9776b86fc689a40dc9a358ec73730 Mon Sep 17 00:00:00 2001
From: Daniel Serpell <daniel.serpell@gmail.com>
Date: Sat, 23 May 2020 13:08:44 -0400
Subject: Adds environment variable to limit available DOS memory.

---
 README.md | 6 ++++++
 src/dbg.c | 5 +++--
 src/dos.c | 7 ++++++-
 src/env.h | 1 +
 4 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/README.md b/README.md
index 47fdd2e..ba878af 100644
--- a/README.md
+++ b/README.md
@@ -55,6 +55,12 @@ The available environment variables are:
                        output, but does not change the DOS NLS information.
                        The default code-page is CP437.
 
+- `EMU2_LOWMEM`        Limits main memory to 512KB, this fixes some old DOS
+                       programs with a bug that checks available memory using
+                       "signed" comparison insructions (JLE instead of JBE).
+                       This is needed at least for MASM versions 1.0 and 1.10.
+
+
 Simple Example
 --------------
 
diff --git a/src/dbg.c b/src/dbg.c
index 11a6dd1..f1c05b3 100644
--- a/src/dbg.c
+++ b/src/dbg.c
@@ -29,9 +29,10 @@ void print_usage(void)
            "  %-18s  DOS current working directory, use 'C:\\' if not given.\n"
            "  %-18s  Set unix path as root of drive 'n', by default all drives\n"
            "\t\t      point to the unix working directory.\n"
-           ". %-18s  Set DOS code-page. Set to '?' to show lost of code-pages.\n",
+           ". %-18s  Set DOS code-page. Set to '?' to show lost of code-pages.\n"
+           ". %-18s  Limit DOS memory to 512KB, fixes some old buggy programs.\n",
            prog_name, ENV_DBG_NAME, ENV_DBG_OPT, ENV_PROGNAME, ENV_DEF_DRIVE, ENV_CWD,
-           ENV_DRIVE "n", ENV_CODEPAGE);
+           ENV_DRIVE "n", ENV_CODEPAGE, ENV_LOWMEM);
     exit(EXIT_SUCCESS);
 }
 
diff --git a/src/dos.c b/src/dos.c
index 84a6816..3d916e5 100644
--- a/src/dos.c
+++ b/src/dos.c
@@ -2052,7 +2052,12 @@ void init_dos(int argc, char **argv)
 
     // Init memory handling - available start address at 0x800,
     // ending address at 0xA0000.
-    mcb_init(0x80, 0xA000);
+    // We limit here memory to less than 512K to fix some old programs
+    // that check memori using "JLE" instead of "JBE".
+    if(getenv(ENV_LOWMEM))
+        mcb_init(0x80, 0x7FFF);
+    else
+        mcb_init(0x80, 0xA000);
 
     // Init SYSVARS
     dos_sysvars = get_static_memory(128, 0);
diff --git a/src/env.h b/src/env.h
index 528a2dc..d4a227b 100644
--- a/src/env.h
+++ b/src/env.h
@@ -8,3 +8,4 @@
 #define ENV_CWD       "EMU2_CWD"
 #define ENV_DRIVE     "EMU2_DRIVE_"
 #define ENV_CODEPAGE  "EMU2_CODEPAGE"
+#define ENV_LOWMEM    "EMU2_LOWMEM"
-- 
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