Changeset 97726 in vbox for trunk/src/VBox/Devices

Timestamp:

Dec 2, 2022 1:01:48 AM (2 years ago)

Author:

vboxsync

Message:

BIOS: The compiler refuses to re-load / restore DS after calling the VGA BIOS init code in rom_scan() because of the small memory model, so do it forcibly as the C compiler assumes DS=f000h everywhere. bugref:6549

File:

• trunk/src/VBox/Devices/PC/BIOS/post.c (modified) (3 diffs)

trunk/src/VBox/Devices/PC/BIOS/post.c

@@ -31 +31 @@
 #include "inlines.h"
 
-#if DEBUG_POST
+#if DEBUG_POST  || 0
 #  define DPRINT(...) BX_DEBUG(__VA_ARGS__)
 #else
@@ -81 +81 @@
 
 /* The ROM init routine might trash register. Give the compiler a heads-up. */
-typedef void (rom_init_rtn)(void);
-#pragma aux rom_init_rtn modify [ax bx cx dx si di es] loadds;
+typedef void __far (rom_init_rtn)(void);
+#pragma aux rom_init_rtn modify [ax bx cx dx si di es /*ignored:*/ bp] /*ignored:*/ loadds;
+
+/* The loadds bit is ignored for rom_init_rtn, the impression from the code
+   generator is that this is due to using a memory model where DS is fixed.
+   If we add DS as a modified register, we'll get run into compiler error
+   E1122 (BAD_REG) because FixedRegs() in cg/intel/386/c/386rgtbl.c returns
+   HW_DS as part of the fixed register set that cannot be modified.
+
+   The problem of the vga bios trashing DS isn't a biggie, except when
+   something goes sideways before we can reload it.  Setting a I/O port
+   breakpoint on port 80h and wait a while before resuming execution
+   (ba i 1 80 "sleep 400 ; g") will usually trigger a keyboard init panic and
+   showcase the issue.  The panic message is garbage because it's read from
+   segment 0c000h instead of 0f000h. */
+static void restore_ds_as_dgroup(void);
+#pragma aux restore_ds_as_dgroup = \
+    "mov ax, 0f000h" \
+    "mov ds, ax" \
+    modify exact [ax];
+
 
 /* Scan for ROMs in the given range and execute their POST code. */
@@ -98 +117 @@
             DPRINT("Found ROM at segment %04X\n", start_seg);
             if (!rom_checksum((void __far *)rom, rom->num_blks)) {
-                rom_init_rtn    __far *rom_init;
+                rom_init_rtn *rom_init;
 
                 /* Checksum good, initialize ROM. */
-                rom_init = (void __far *)&rom->code;
+                rom_init = (rom_init_rtn *)&rom->code;
                 rom_init();
                 int_disable();
+                restore_ds_as_dgroup();
+                /** @todo BP is not restored. */
                 DPRINT("ROM initialized\n");