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/* $OpenBSD: start.s,v 1.2 1996/04/26 18:23:12 mickey Exp $ */
/* $NetBSD: start.s,v 1.4 1994/10/27 04:21:25 cgd Exp $ */
#include "asm.h"
/* At entry, the processor is in 16 bit real mode and the code is being
* executed from an address it was not linked to. Code must be pic and
* 32 bit sensitive until things are fixed up.
*/
.word 0xaa55 /* bios extension signature */
.byte 0 /* no. of 512B blocks */
jmp 1f /* enter from bios here */
.byte 0 /* checksum */
ENTRY(start)
1:
cli
/* save the bios return address in these registers until protected
* mode %ds is set up
* mov (%esp), %edx
* mov 2(%esp), %ebp
*/
pop %dx /* return offset */
pop %ebp /* return segment */
/* save stack [, data] context in case we are under dos */
mov %esp, %ecx
mov %ss, %ax
mov %eax, %ebx
/* set up a usable stack */
.byte 0xb8 /* (mov $0xa000, %ax) */
.word 0xa000
mov %ax, %ss
xor %esp, %esp
push %ebp /* return segment */
push %dx /* return offset */
push %ds
#if notdef
jmp ret16
#endif
#ifdef CHECK_386
/*
* check if 386 or later
* from Intel i486 programmer's reference manual, section 22.10
* Care must be taken with the first few instructions to ensure
* operations are compatible with 386 progenitors - no operand
* or address size overrides, all operations must be 16 bit.
* Real mode not well supported by gas so it looks a bit crufty
*
* TBD - there is little stack space, although the routine below
* uses only two bytes; might set up new stack first thing, then
* check processor type - would mean carrying sp, ss in two gp
* registers for a while. also make alternate provisions for saving
* ds: below.
*/
pushf
pop %ebx
.byte 0x81, 0xe3 /* (and 0x0fff, %ebx) */
.word 0x0fff
push %ebx
popf
pushf
pop %eax
.byte 0x25 /* (and 0xf000, %eax) */
.word 0xf000
.byte 0x3d /* (cmp 0xf000, %eax) */
.word 0xf000
jz Lwrong_cpu /* \'86 */
.byte 0x81, 0xcb /* (or 0xf000, %ebx) */
.word 0xf000
push %ebx
popf
pushf
pop %eax
.byte 0x25 /* (and 0xf000, %eax) */
.word 0xf000
jnz Lcpu_ok /* else is \'286 */
Lwrong_cpu:
.byte 0xbb /* (mov bad_cpu_msg, %ebx) */
.word bad_cpu_msg
.byte 0xe8 /* (call xputs) */
.word xputs-.-2
lret
xputc: /* print byte in %al */
data32
pusha
.byte 0xbb /* (mov $0x1, %ebx) %bh=0, %bl=1 (blue) */
.word 0x0001
movb $0xe, %ah
/* sti */
int $0x10 /* display a byte */
/* cli */
data32
popa
ret
xputs: /* print string pointed to by cs:bx */
data32
pusha
1:
cs
.byte 0x8a, 0x07 /* (mov (%ebx), %al) */
cmpb $0, %al
jz 1f
push %ebx
.byte 0xe8 /* (call xputc) */
.word xputc-.-2
pop %ebx
inc %ebx
jmp 1b
1:
data32
popa
ret
bad_cpu_msg: .asciz "netboot: cpu cannot execute '386 instructions, net boot not done.\n\r"
Lcpu_ok:
/*
* at this point it is known this can execute 386 instructions
* so operand and address size prefixes are ok
*/
#endif /* CHECK_386 */
/* copy rom to link addr, prepare for relocation */
xor %esi, %esi /* source */
opsize
mov $0, %edi /* destination */
opsize
mov $(RELOC)>>4, %eax
mov %ax, %es
opsize
mov $(ROM_SIZE), %ecx /* count */
cs
rep
movsb
addrsize
cs
lgdt gdtarg-RELOC
/* turn on protected mode */
cli
mov %cr0, %eax
opsize
or $CR0_PE, %eax
mov %eax, %cr0
/* jump to relocation, flush prefetch queue, and reload %cs */
opsize
ljmp $KERN_CODE_SEG, $1f
1:
/* reload other segment registers */
movl $KERN_DATA_SEG, %eax
movl %ax, %ds
movl %ax, %es
movl %ax, %ss
movl $0xa0000, %esp
call _main
call _exit
_ExitToBios:
.globl _ExitToBios
/* set up a dummy stack frame for the second seg change. */
mov $(RELOC)>>4, %eax
pushw %ax /* real cs */
pushw $2f /* real pc */
/* Change to use16 mode. */
ljmp $BOOT_16_SEG, $1f /* jump to a 16 bit segment */
1:
/* clear the PE bit of CR0 */
mov %cr0, %eax
opsize
and $0!CR0_PE, %eax
mov %eax, %cr0
/* make intersegment jmp to flush the processor pipeline
* using the fake stack frame set up earlier
* and reload CS register
*/
lret
2:
/* we are in real mode now
* set up the real mode segment registers : DS, SS, ES
*/
movw %cs, %ax
movw %ax, %ds
movw %ax, %ss
movw %ax, %es
ret16: /* temporary label - remove (TBD) */
/* now in dumbed down mode, caveats */
/* restore old context and return to whatever called us */
pop %ds
pop %dx
pop %ebp
mov %ebx, %eax
mov %ax, %ss
mov %ecx, %esp
push %ebp
push %dx
sti
lret
#ifdef USE_BIOS
_real_to_prot:
.global _real_to_prot
addrsize
cs
lgdt gdtarg-RELOC
cli
mov %cr0, %eax
opsize
or $CR0_PE, %eax
mov %eax, %cr0
/* jump to relocation, flush prefetch queue, and reload %cs */
opsize
ljmp $KERN_CODE_SEG, $1f
1:
movl $KERN_DATA_SEG, %eax
movl %ax, %ds
movl %ax, %es
movl %ax, %ss
ret
#endif
#ifdef USE_BIOS
_prot_to_real:
.global _prot_to_real
/* set up a dummy stack frame for the second seg change. */
movl $(RELOC), %eax
sarl $4, %eax
pushw %ax /* real cs */
pushw $2f /* real pc */
/* Change to use16 mode. */
ljmp $BOOT_16_SEG, $1f /* jump to a 16 bit segment */
1:
/* clear the PE bit of CR0 */
mov %cr0, %eax
opsize
and $0!CR0_PE, %eax
mov %eax, %cr0
/* make intersegment jmp to flush the processor pipeline
* using the fake stack frame set up earlier
* and reload CS register
*/
lret
2:
/* we are in real mode now
* set up the real mode segment registers : DS, SS, ES
*/
movw %cs, %ax
movw %ax, %ds
movw %ax, %ss
movw %ax, %es
opsize
ret
#endif
.align 4
gdt:
.word 0, 0
.byte 0, 0x00, 0x00, 0
/* code segment */
.word 0xffff, 0
.byte 0, 0x9f, 0xcf, 0
/* data segment */
.word 0xffff, 0
.byte 0, 0x93, 0xcf, 0
/* 16 bit real mode */
.word 0xffff, 0
.byte 0, 0x9e, 0x00, 0
.align 4
gdtarg:
.word 0x1f /* limit */
.long gdt /* addr */
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