memdisk: relocate real-mode code before booting

Relocate the real-mode code before booting.  This allows the target
bootstrap to be loaded at an arbitrary address, not necessarily
0x7c00, and to be almost arbitrarily long.

Add some initial infrastructure for other bootstrap addresses, too.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 86 insertions, 28 deletions

diff --git a/memdisk/setup.c b/memdisk/setup.c
index f9b9ed1f..a46b12e6 100644
--- a/memdisk/setup.c
+++ b/memdisk/setup.c
@@ -163,10 +163,14 @@ struct real_mode_args {
     uint32_t rm_bounce;
     uint32_t rm_base;
     uint32_t rm_handle_interrupt;
+    uint32_t rm_gdt;
     uint32_t rm_size;
 };
 struct real_mode_args rm_args;
 
+__cdecl syscall_t syscall;
+void *sys_bounce;
+
 /* Access to high memory */
 
 /* Access to objects in the zero page */
@@ -348,9 +352,7 @@ void unzip_if_needed(uint32_t * where_p, uint32_t * size_p)
 		printf("Moving compressed data from 0x%08x to 0x%08x\n",
 		       where, newwhere);
 
-		/* Our memcpy() is OK, because we always move from a higher
-		   address to a lower one */
-		memcpy((void *)newwhere, (void *)where, size);
+		memmove((void *)newwhere, (void *)where, size);
 		where = newwhere;
 	    }
 
@@ -670,9 +672,9 @@ const struct geometry *get_disk_image_geometry(uint32_t where, uint32_t size)
  */
 void __attribute__ ((noreturn)) die(void)
 {
-    asm volatile ("sti");
+    sti();
     for (;;)
-	asm volatile ("hlt");
+	asm volatile("hlt");
 }
 
 /*
@@ -703,6 +705,58 @@ static uint32_t pnp_install_check(void)
     return 0;
 }
 
+static void update_global_vars(void)
+{
+    syscall = (__cdecl syscall_t) rm_args.rm_syscall;
+    sys_bounce = (void *)rm_args.rm_bounce;
+    shdr = (void *)rm_args.rm_base;
+}
+
+/*
+ * Relocate the real-mode code to a new segment
+ */
+struct gdt_ptr {
+    uint16_t limit;
+    uint32_t base;
+} __attribute__((packed));
+
+static void set_seg_base(uint32_t gdt_base, int seg, uint32_t v)
+{
+    *(uint16_t *)(gdt_base + seg + 2) = v;
+    *(uint8_t *)(gdt_base + seg + 4) = v >> 16;
+    *(uint8_t *)(gdt_base + seg + 7) = v >> 24;
+}
+
+static void relocate_rm_code(uint32_t newbase)
+{
+    uint32_t gdt_base;
+    uint32_t oldbase = rm_args.rm_base;
+    uint32_t delta   = newbase - oldbase;
+
+    cli();
+    memmove((void *)newbase, (void *)oldbase, rm_args.rm_size);
+
+    rm_args.rm_return  		+= delta;
+    rm_args.rm_syscall 		+= delta;
+    rm_args.rm_bounce  		+= delta;
+    rm_args.rm_base    		+= delta;
+    rm_args.rm_gdt              += delta;
+    rm_args.rm_handle_interrupt	+= delta;
+
+    gdt_base = rm_args.rm_gdt;
+
+    *(uint32_t *)(gdt_base+2)    = gdt_base;	/* GDT self-pointer */
+
+    /* Segments 0x10 and 0x18 are real-mode-based */
+    set_seg_base(gdt_base, 0x10, rm_args.rm_base);
+    set_seg_base(gdt_base, 0x18, rm_args.rm_base);
+
+    asm volatile("lgdtl %0" : : "m" (*(char *)gdt_base));
+    sti();
+
+    update_global_vars();
+}
+
 #define STACK_NEEDED	512	/* Number of bytes of stack */
 
 /*
@@ -710,9 +764,6 @@ static uint32_t pnp_install_check(void)
  * Returns the drive number (which is then passed in %dl to the
  * called routine.)
  */
-__cdecl syscall_t syscall;
-void *sys_bounce;
-
 void setup(const struct real_mode_args *rm_args_ptr)
 {
     unsigned int bin_size;
@@ -727,18 +778,20 @@ void setup(const struct real_mode_args *rm_args_ptr)
     int total_size, cmdlinelen;
     com32sys_t regs;
     uint32_t ramdisk_image, ramdisk_size;
+    uint32_t boot_base, rm_base;
     int bios_drives;
     int do_edd = 1;		/* 0 = no, 1 = yes, default is yes */
     int no_bpt;			/* No valid BPT presented */
+    uint32_t boot_seg = 0;	/* Meaning 0000:7C00 */
+    uint32_t boot_len = 512;	/* One sector */
+    uint32_t boot_lba = 0;	/* LBA of bootstrap code */
 
     /* We need to copy the rm_args into their proper place */
     memcpy(&rm_args, rm_args_ptr, sizeof rm_args);
     sti();			/* ... then interrupts are safe */
 
     /* Set up global variables */
-    syscall = (__cdecl syscall_t) rm_args.rm_syscall;
-    sys_bounce = (void *)rm_args.rm_bounce;
-    shdr = (void *)rm_args.rm_base;
+    update_global_vars();
 
     /* Show signs of life */
     printf("%s  %s\n", memdisk_version, copyright);
@@ -1056,9 +1109,8 @@ void setup(const struct real_mode_args *rm_args_ptr)
 	wrz_8(BIOS_EQUIP, equip);
 
 	/* Install DPT pointer if this was the only floppy */
-	if (getcmditem("dpt") != CMD_NOTFOUND || ((nflop == 1 || no_bpt)
-						  && getcmditem("nodpt") ==
-						  CMD_NOTFOUND)) {
+	if (getcmditem("dpt") != CMD_NOTFOUND ||
+	    ((nflop == 1 || no_bpt) && getcmditem("nodpt") == CMD_NOTFOUND)) {
 	    /* Do install a replacement DPT into INT 1Eh */
 	    pptr->dpt_ptr = hptr->patch_offs + offsetof(struct patch_area, dpt);
 	}
@@ -1076,21 +1128,29 @@ void setup(const struct real_mode_args *rm_args_ptr)
     printf("new: int13 = %08x  int15 = %08x  int1e = %08x\n",
 	   rdz_32(BIOS_INT13), rdz_32(BIOS_INT15), rdz_32(BIOS_INT1E));
 
-    /* Reboot into the new "disk"; this is also a test for the interrupt hooks */
-    puts("Loading boot sector... ");
-
-    memset(&regs, 0, sizeof regs);
-    // regs.es = 0;
-    regs.eax.w[0] = 0x0201;	/* Read sector */
-    regs.ebx.w[0] = 0x7c00;	/* 0000:7C00 */
-    regs.ecx.w[0] = 1;		/* One sector */
-    regs.edx.w[0] = geometry->driveno;
-    syscall(0x13, &regs, &regs);
+    /* Figure out entry point */
+    if (!boot_seg) {
+	boot_base  = 0x7c00;
+	shdr->sssp = 0x7c00; 
+	shdr->csip = 0x7c00; 
+    } else {
+	boot_base  = boot_seg << 4;
+	shdr->sssp = boot_seg << 16;
+	shdr->csip = boot_seg << 16;
+    }
 
-    if (regs.eflags.l & 1) {
-	puts("MEMDISK: Failed to load new boot sector\n");
+    /* Relocate the real-mode code to below the stub */
+    rm_base = (driveraddr - rm_args.rm_size) & ~15;
+    if (rm_base < boot_base + boot_len) {
+	puts("MEMDISK: bootstrap too large to load\n");
 	die();
     }
+    relocate_rm_code(rm_base);
+
+    /* Reboot into the new "disk" */
+    puts("Loading boot sector... ");
+
+    memcpy((void *)boot_base, (char *)pptr->diskbuf + boot_lba*512, boot_len);
 
     if (getcmditem("pause") != CMD_NOTFOUND) {
 	puts("press any key to boot... ");
@@ -1103,6 +1163,4 @@ void setup(const struct real_mode_args *rm_args_ptr)
     /* On return the assembly code will jump to the boot vector */
     shdr->esdi = pnp_install_check();
     shdr->edx  = geometry->driveno;
-    shdr->sssp = 0x7c00; 
-    shdr->csip = 0x7c00; 
 }