mem: don't count reserved memory as used upper memory

Upper memory was a fixed 256K region, 0xA000-0xDFFF, with used taken
as that total minus the free blocks found in the chain. Anything in
the region that the upper memory chain does not reach - video memory,
option ROMs, the BIOS - was therefore reported as in use by somebody,
although DOS can neither give it out nor account for it.

Take the upper total from the chain instead, as FreeDOS MEM does: the
blocks themselves, allocated and free, wherever they sit, and the rest
of the area above 640K is the reserved region, whose size is now what
is left over rather than a fixed 128K.

Measured under dosemu2 with the FreeDOS MEM.EXE beside our own, which
is what dosemu2's test/func_comcom_internal.py compares. With a 32K
hole in the chain, which `$_umb_b0 = (0)` makes:

                      before            after      FreeDOS MEM.EXE
  Upper      256K  120K  136K    224K  88K  136K    224K  88K  136K
  Reserved   128K  128K    0K    160K 160K    0K    160K 160K    0K

The two agreed in the default configuration only by accident: the 36K
hole the chain skips at 0xB800 happens to be the size of the free area
it reports above 0xE000, and the two errors cancelled. With the holes
of any other size, the fixed total charged the difference to "used".

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

src/mem.c

@@ -53,17 +53,15 @@
 #define CONV_MAX_KB (UMB_START_SEG / PARAS_PER_KB)
 
 /*
- * Above conventional memory, FreeDOS MEM splits the first megabyte the
- * way MS-DOS MEM did: 0xA000-0xDFFF is "Upper", and 0xE000-0xFFFF is
- * "Reserved" - the BIOS and option ROM area, reported as entirely in
- * use. Both are fixed-size regions, not sums of the blocks found in
- * them, and free UMBs count towards upper memory wherever they sit.
- * Report them the same way, so our numbers can be compared with
- * MEM.EXE's directly.
+ * Above conventional memory, FreeDOS MEM splits what it finds in two:
+ * the blocks of the upper memory chain are "Upper", and everything else
+ * below 1 MB is "Reserved" - video memory, option ROMs and the BIOS -
+ * reported as entirely in use. So upper memory is the sum of the blocks
+ * themselves, wherever they sit, and the area the chain does not reach
+ * is reserved rather than used. Report them the same way, so our
+ * numbers can be compared with MEM.EXE's directly.
  */
-#define RESERVED_START_SEG 0xE000
-#define UMB_TOTAL_KB ((RESERVED_START_SEG - UMB_START_SEG) / PARAS_PER_KB)
-#define RESERVED_TOTAL_KB ((0x10000 - RESERVED_START_SEG) / PARAS_PER_KB)
+#define UPPER_AREA_KB ((0x10000 - UMB_START_SEG) / PARAS_PER_KB)
 
 /* Rounds to nearest, as FreeDOS MEM does. */
 static uint32_t paras_to_kb(uint32_t paras)
@@ -572,6 +570,7 @@ void perform_mem(const char *arg)
   uint32_t conv_free_paras = 0;
   uint32_t conv_largest_paras = 0;
 
+  uint32_t umb_total_paras = 0;
   uint32_t umb_free_paras = 0;
   uint32_t umb_largest_paras = 0;
 
@@ -662,7 +661,12 @@ void perform_mem(const char *arg)
 
     /* the MCB header itself belongs to the region it sits in */
     if (!is_umb)
+    {
       conv_total_paras += conv_paras + 1;
+      umb_total_paras += umb_paras;
+    }
+    else
+      umb_total_paras += umb_paras + 1;
 
     if (mcb.owner_psp == 0)
     {
@@ -789,10 +793,13 @@ void perform_mem(const char *arg)
   uint32_t conv_free_kb = paras_to_kb(conv_free_paras);
   uint32_t conv_used_kb = (conv_total_kb > conv_free_kb) ?
     (conv_total_kb - conv_free_kb) : 0;
-  uint32_t umb_total_kb = UMB_TOTAL_KB;
+  uint32_t umb_total_kb = paras_to_kb(umb_total_paras);
   uint32_t umb_free_kb = paras_to_kb(umb_free_paras);
   uint32_t umb_used_kb = (umb_total_kb > umb_free_kb) ?
     (umb_total_kb - umb_free_kb) : 0;
+  /* what the chain never reached is reserved, not used upper memory */
+  uint32_t reserved_total_kb = (UPPER_AREA_KB > umb_total_kb) ?
+    (UPPER_AREA_KB - umb_total_kb) : 0;
 
   if (opt_classify)
   {
@@ -924,7 +931,7 @@ void perform_mem(const char *arg)
            "Upper", str_tot, str_used, str_free);
   print_line(line_buf);
 
-  format_kb(RESERVED_TOTAL_KB, str_tot, sizeof(str_tot));
+  format_kb(reserved_total_kb, str_tot, sizeof(str_tot));
   format_kb(0, str_free, sizeof(str_free));
   snprintf(line_buf, sizeof(line_buf), "%-16s  %8s   %8s   %8s",
            "Reserved", str_tot, str_tot, str_free);
@@ -942,9 +949,9 @@ void perform_mem(const char *arg)
 
   print_line("----------------  --------   --------   --------");
 
-  format_kb(conv_total_kb + umb_total_kb + RESERVED_TOTAL_KB + xms_total_kb,
+  format_kb(conv_total_kb + umb_total_kb + reserved_total_kb + xms_total_kb,
             str_tot, sizeof(str_tot));
-  format_kb(conv_used_kb + umb_used_kb + RESERVED_TOTAL_KB + xms_used_kb,
+  format_kb(conv_used_kb + umb_used_kb + reserved_total_kb + xms_used_kb,
             str_used, sizeof(str_used));
   format_kb(conv_free_kb + umb_free_kb + xms_free_kb, str_free, sizeof(str_free));
   snprintf(line_buf, sizeof(line_buf), "%-16s  %8s   %8s   %8s",