MichaelPetch wrote:Your VBE structure will have to be below 0x100000 to properly work since BIOS interrupts will have difficulty writing above the 1MiB mark. As it is your VBE structure is on the stack where ESP > 0x100000. Pick a memory location in lower memory (like 0x600) that is above the BIOS Data area and the interrupt vector table. This address can be used for both protected mode and realmode. Your
OFF and
SEG macros assume realmode FAR pointers. Adjust them so they compute to a normalized segment:offset for physical address < 0x100000. The macros could look like:
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#define SEG(FP) ((uint16_t)(((unsigned long)FP)>>4))
#define OFF(FP) ((uint16_t)(((unsigned long)FP)&0xF))
Your VBE function could look like:
Code: Select all
int VBE( VBE_INFO* vbe )
{
regs16_t regs;
regs.ax = 0x4F00;
regs.es = SEG(vbe);
regs.di = OFF(vbe);
int32( 0x10, ®s );
if ( regs.ax == 0x004F )
return 1;
return 0;
}
Your
main could look like
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int main( void ) {
VBE_INFO *vbe = (VBE_INFO *)0x600;
int res;
vbe->signature[0] = 'V';
vbe->signature[1] = 'B';
vbe->signature[2] = 'E';
vbe->signature[3] = '2';
res = VBE( vbe );
if (res == 1)
{
if(vbe->version == 0x0000) // this is what I get always
printk('N');
else if(vbe->version != 0x0000)
printk('Y');
}
else
printk('0');
}
Function
int32 assumes that the interrupts were already on prior to being called and turns them on when finished. If you don't have a protected mode IDT set up (or you haven't disabled all the IRQs on the PICs) the code will fault in protected mode when
int32 is finished. I've amended the code to only re-enable interrupts if they were already enabled when first called:
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[bits 32]
global int32, _int32
struc regs16_t
.di resw 1
.si resw 1
.bp resw 1
.sp resw 1
.bx resw 1
.dx resw 1
.cx resw 1
.ax resw 1
.gs resw 1
.fs resw 1
.es resw 1
.ds resw 1
.ef resw 1
endstruc
%define INT32_BASE 0x7C00 ; base BIOS address
%define REBASE(x) (((x) - reloc) + INT32_BASE)
%define GDTENTRY(x) ((x) << 3)
%define CODE32 GDTENTRY(1) ; 0x08
%define DATA32 GDTENTRY(2) ; 0x10
%define CODE16 GDTENTRY(3) ; 0x18
%define DATA16 GDTENTRY(4) ; 0x20
%define STACK16 (INT32_BASE - regs16_t_size)
section .text
int32: use32
_int32:
pushf
pop dword [eflags] ; Save the current CPU flags (including IF state)
cli ; disable interrupts
pusha ; save register state to 32bit stack
mov esi, reloc ; set source to code below
mov edi, INT32_BASE ; set destination to new base address
mov ecx, (int32_end - reloc) ; set copy size to our codes size: LOOK AT THE COMMENT TAGGED WITH %*%
cld ; clear direction flag (so we copy forward)
rep movsb ; do the actual copy (relocate code to low 16bit space)
jmp INT32_BASE ; jump to new code location
reloc: use32 ; %*% From here on gets copy startting from INT32_BASE
mov [REBASE(stack32_ptr)], esp ; save 32bit stack pointer
sidt [REBASE(idt32_ptr)] ; save 32bit idt pointer
sgdt [REBASE(gdt32_ptr)] ; save 32bit gdt pointer
lgdt [REBASE(gdt16_ptr)] ; load 16bit gdt pointer
lea esi, [esp+0x24] ; set position of intnum on 32bit stack
lodsd ; read intnum into eax
mov [REBASE(ib)], al ; set intrrupt immediate byte from our arguments
mov esi, [esi] ; read regs pointer in esi as source
mov edi, STACK16 ; set destination to 16bit stack
mov ecx, regs16_t_size ; set copy size to our struct size
mov esp, edi ; save destination to as 16bit stack offset
rep movsb ; do the actual copy (32bit stack to 16bit stack)
jmp word CODE16:REBASE(p_mode16) ; switch to 16bit selector (16bit protected mode)
p_mode16: use16
mov ax, DATA16 ; get our 16bit data selector
mov ds, ax ; set ds to 16bit selector
mov es, ax ; set es to 16bit selector
mov fs, ax ; set fs to 16bit selector
mov gs, ax ; set gs to 16bit selector
mov ss, ax ; set ss to 16bit selector
mov eax, cr0 ; get cr0 so we can modify it
and al, ~0x01 ; mask off PE bit to turn off protected mode
mov cr0, eax ; set cr0 to result
jmp word 0x0000:REBASE(r_mode16) ; finally set cs:ip to enter real-mode
r_mode16: use16
xor ax, ax ; set ax to zero
mov ds, ax ; set ds so we can access idt16
mov ss, ax ; set ss so they the stack is valid
lidt [REBASE(idt16_ptr)] ; load 16bit idt
popa ; load general purpose registers from 16bit stack
pop gs ; load gs from 16bit stack
pop fs ; load fs from 16bit stack
pop es ; load es from 16bit stack
pop ds ; load ds from 16bit stack
sti ; enable interrupts
db 0xCD ; opcode of INT instruction with immediate byte
ib: db 0x00
cli ; disable interrupts
xor sp, sp ; zero sp so we can reuse it
mov ss, sp ; set ss so the stack is valid
mov sp, INT32_BASE ; set correct stack position so we can copy back
pushf ; save eflags to 16bit stack
push ds ; save ds to 16bit stack
push es ; save es to 16bit stack
push fs ; save fs to 16bit stack
push gs ; save gs to 16bit stack
pusha ; save general purpose registers to 16bit stack
mov eax, cr0 ; get cr0 so we can modify it
inc eax ; set PE bit to turn on protected mode
mov cr0, eax ; set cr0 to result
jmp dword CODE32:REBASE(p_mode32) ; switch to 32bit selector (32bit protected mode)
p_mode32: use32
mov ax, DATA32 ; get our 32bit data selector
mov ds, ax ; reset ds selector
mov es, ax ; reset es selector
mov fs, ax ; reset fs selector
mov gs, ax ; reset gs selector
mov ss, ax ; reset ss selector
lgdt [REBASE(gdt32_ptr)] ; restore 32bit gdt pointer
lidt [REBASE(idt32_ptr)] ; restore 32bit idt pointer
mov esp, [REBASE(stack32_ptr)] ; restore 32bit stack pointer
mov esi, STACK16 ; set copy source to 16bit stack
lea edi, [esp+0x28] ; set position of regs pointer on 32bit stack
mov edi, [edi] ; use regs pointer in edi as copy destination
mov ecx, regs16_t_size ; set copy size to our struct size
cld ; clear direction flag (so we copy forward)
rep movsb ; do the actual copy (16bit stack to 32bit stack)
popa ; restore registers
and dword [eflags], 1<<9 ; If original interrupt state was on, issue STI
jz int_off ; otherwise keep interrupts off
sti
int_off:
ret ; return to caller
stack32_ptr: ; address in 32bit stack after we
dd 0x00000000 ; save all general purpose registers
idt32_ptr: ; IDT table pointer for 32bit access
dw 0x0000 ; table limit (size)
dd 0x00000000 ; table base address
gdt32_ptr: ; GDT table pointer for 32bit access
dw 0x0000 ; table limit (size)
dd 0x00000000 ; table base address
idt16_ptr: ; IDT table pointer for 16bit access
dw 0x03FF ; table limit (size)
dd 0x00000000 ; table base address
gdt16_base: ; GDT descriptor table
.null: ; 0x00 - null segment descriptor
dd 0x00000000 ; must be left zero'd
dd 0x00000000 ; must be left zero'd
.code32: ; 0x01 - 32bit code segment descriptor 0xFFFFFFFF
dw 0xFFFF ; limit 0:15
dw 0x0000 ; base 0:15
db 0x00 ; base 16:23
db 0x9A ; present, iopl/0, code, execute/read
db 0xCF ; 4Kbyte granularity, 32bit selector; limit 16:19
db 0x00 ; base 24:31
.data32: ; 0x02 - 32bit data segment descriptor 0xFFFFFFFF
dw 0xFFFF ; limit 0:15
dw 0x0000 ; base 0:15
db 0x00 ; base 16:23
db 0x92 ; present, iopl/0, data, read/write
db 0xCF ; 4Kbyte granularity, 32bit selector; limit 16:19
db 0x00 ; base 24:31
.code16: ; 0x03 - 16bit code segment descriptor 0x000FFFFF
dw 0xFFFF ; limit 0:15
dw 0x0000 ; base 0:15
db 0x00 ; base 16:23
db 0x9A ; present, iopl/0, code, execute/read
db 0x0F ; 1Byte granularity, 16bit selector; limit 16:19
db 0x00 ; base 24:31
.data16: ; 0x04 - 16bit data segment descriptor 0x000FFFFF
dw 0xFFFF ; limit 0:15
dw 0x0000 ; base 0:15
db 0x00 ; base 16:23
db 0x92 ; present, iopl/0, data, read/write
db 0x0F ; 1Byte granularity, 16bit selector; limit 16:19
db 0x00 ; base 24:31
gdt16_ptr: ; GDT table pointer for 16bit access
dw gdt16_ptr - gdt16_base - 1 ; table limit (size)
dd gdt16_base ; table base address
eflags: dd 0 ; Storage for EFLAGS (including IF)
int32_end: ; end marker (so we can copy the code)
Thanks MichaelPetch for your comprehensive comment and code. Regarding the fact that
int32, in case of lack of p-mode IDT table would make a fault, I confirm that. Since I wan exactly aware of what might have been the source of problem, for now, I just ignored it. With your updated code of int32, now, I got the reason for such a fault and learnd how to fix it.
With the use of direct memory assigning for VBE_INFO struct, say at 0x600, the in32 call now is successful.
This is another new fact that I learned from this point
. Now it arises another inquiry:
based on the structure below, there are quite some free bytes below 1 Mb. Could it be always free?, so once in protected mode I can assign memory blocks for structs such as VESA-related structs?
0x00000 - 0x003FF - Real Mode IVT
0x00400 - 0x004FF - BIOS Data Area
0x00500 - 0x07BFF - Unused
0x07C00 - 0x07DFF - Bootloader
0x07E00 - 0x9FFFF - Unused
0xA0000 - 0xBFFFF - Video RAM
.
.
.
Cheers.
Iman.
