Booting VxWorks from LinuxBios

[Christopher Stutts]

Asm part of jump from Linuxbios to vxWorks


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//
//  cpmasm.S
//
//  Performs real-mode switch and then jump to code copied to flash at 0x800:0.
//  (VxWorks startup code expects a 0 offset, so 0:0x8000 is verboten.)
//  Mode switch is two part: loading all seg registers with 16-bit segments, then
//  jumping to realmode segment.  GDT in intel_start32.S contains a 16-bit data segment
//  0x30 & a 16-bit code segment 0x28.  0x28:0 = 0xf0000 physical.  The jump to
//  0x28:offset_patch_address requires a far jump with a 16-bit offset, which the assembler
//  doesn't do.  The offset there fore is patched before we get executed by some C code.


 	.text
//	.code32

    .global   flashOSBootasm
flashOSBootasm:

//Load all data segment regs with a 16-bit segment
        mov  $0x30,%ax
        mov  %ax,%ds
        mov  %ax,%es
        mov  %ax,%fs
        mov  %ax,%gs
        mov  %ax,%ss

//Jump to a 16-bit code segment
        ljmp $0x28,$0x00000000

.global offset_patch_address
.long offset_patch_address
offset_patch_address:

		movl %cr0, %eax          //mov eax,cr0
		and  $0xfffffffe,%eax    //and eax,0ffffffFEh           
		movl %eax,%cr0  //Switch back to real-mode without resetting
        
.global PROTECTION_DISABLED
PROTECTION_DISABLED:

.code16

//Load all data segments regs with 0 until realmode code sets them
        mov  $0,%ax
        nop
        nop
        mov  %ax,%ds
        mov  %ax,%es
        mov  %ax,%fs
        mov  %ax,%ss
        mov  %ax,%gs

//opcode for real-mode jump to 800:0
        .byte 0xea,0,0,0,0x8
	.code32