where [BITS 32] directive?
in [ASM]
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From: opexoc on 13 Aug 2007 04:15 Hello, look at some piece of code which is to be booted by BIOS at startup and enter CPU into protected mode: 1[BITS 16] ; We need 16-bit intructions for Real mode 2 3 [ORG 0x7C00] ; The BIOS loads the boot sector into memory location 0x7C00 4 5 cli ; Disable interrupts, we want to be alone 6 7 xor ax, ax 8 mov ds, ax ; Set DS-register to 0 - used by lgdt 9 10 lgdt [gdt_desc] ; Load the GDT descriptor 11 12 mov eax, cr0 ; Copy the contents of CR0 into EAX 13 or eax, 1 ; Set bit 0 14 mov cr0, eax 15 ; Copy the contents of EAX into CR0 16 ;[BITS 32] 17 jmp 08h:clear_pipe ; Jump to code segment, offset clear_pipe 18 [BITS 32] 19 clear_pipe: 20 mov ax, 10h ; Save data segment identifyer 21 mov ds, ax In which location directive [BITS 32] should appear ( first or second )? When I use first location then computer hang on and doesn't execute properly code in clear_pipe. When I change first bit in cr0 register then I enter into protected mode so I should use 32 bit instruction. Despite of this fact only when I use second directive [BITS 32] ( without first ) everything works ok. Wiktor
From: Frank Kotler on 13 Aug 2007 04:54 opexoc(a)gmail.com wrote: > Hello, > > look at some piece of code which is to be booted by BIOS at startup > and enter CPU into protected mode: > > 1[BITS 16] ; We need 16-bit intructions for Real mode > 2 > 3 [ORG 0x7C00] ; The BIOS loads the boot sector into memory > location 0x7C00 4 > 5 cli ; Disable interrupts, we want to > be alone > 6 > 7 xor ax, ax > 8 mov ds, ax ; Set DS-register to 0 - used by > lgdt > 9 > 10 lgdt [gdt_desc] ; Load the GDT descriptor > 11 > 12 mov eax, cr0 ; Copy the contents of CR0 into > EAX > 13 or eax, 1 ; Set bit 0 > 14 mov cr0, eax > 15 ; Copy the contents of EAX into > CR0 > 16 ;[BITS 32] > 17 jmp 08h:clear_pipe ; Jump to code segment, offset > clear_pipe > 18 [BITS 32] > 19 clear_pipe: > 20 mov ax, 10h ; Save data segment identifyer > 21 mov ds, ax > > In which location directive [BITS 32] should appear ( first or > second )? When I use first location then computer hang on and doesn't > execute properly code in clear_pipe. When I change first bit in cr0 > register then I enter into protected mode so I should use 32 bit > instruction. Despite of this fact only when I use second directive > [BITS 32] ( without first ) everything works ok. Well... the latter is correct. While you may be in "protected mode" after setting cr0, it's a bit in the descriptor loaded into cs that puts the CPU into 32-bit mode... so you're still in 16-bit mode until cs is loaded by the far jump. (I *think* that's the explanation...) Best, Frank
From: //o//annabee on 13 Aug 2007 06:21 P� Mon, 13 Aug 2007 09:54:22 +0100, skrev Frank Kotler <fbkotler(a)verizon.net>: > Well... the latter is correct. While you may be in "protected mode" > after setting cr0, it's a bit in the descriptor loaded into cs that puts > the CPU into 32-bit mode... so you're still in 16-bit mode until cs is > loaded by the far jump. (I *think* that's the explanation...) http://www.youtube.com/watch?v=cxzs46Nxohk&playnext=1 Whats this talk about NAU Frank. Do you know? > Best, > Frank
From: Rod Pemberton on 13 Aug 2007 05:32 "Frank Kotler" <fbkotler(a)verizon.net> wrote in message news:2%Uvi.1429$hK5.87(a)trndny02... > opexoc(a)gmail.com wrote: > > Hello, > > > > look at some piece of code which is to be booted by BIOS at startup > > and enter CPU into protected mode: > > > > 1[BITS 16] ; We need 16-bit intructions for Real mode > > 2 > > 3 [ORG 0x7C00] ; The BIOS loads the boot sector into memory > > location 0x7C00 4 > > 5 cli ; Disable interrupts, we want to > > be alone > > 6 > > 7 xor ax, ax > > 8 mov ds, ax ; Set DS-register to 0 - used by > > lgdt > > 9 > > 10 lgdt [gdt_desc] ; Load the GDT descriptor > > 11 > > 12 mov eax, cr0 ; Copy the contents of CR0 into > > EAX > > 13 or eax, 1 ; Set bit 0 > > 14 mov cr0, eax > > 15 ; Copy the contents of EAX into > > CR0 > > 16 ;[BITS 32] > > 17 jmp 08h:clear_pipe ; Jump to code segment, offset > > clear_pipe > > 18 [BITS 32] > > 19 clear_pipe: > > 20 mov ax, 10h ; Save data segment identifyer > > 21 mov ds, ax > > > > In which location directive [BITS 32] should appear ( first or > > second )? When I use first location then computer hang on and doesn't > > execute properly code in clear_pipe. When I change first bit in cr0 > > register then I enter into protected mode so I should use 32 bit > > instruction. Despite of this fact only when I use second directive > > [BITS 32] ( without first ) everything works ok. > > Well... the latter is correct. While you may be in "protected mode" > after setting cr0, it's a bit in the descriptor loaded into cs that puts > the CPU into 32-bit mode... so you're still in 16-bit mode until cs is > loaded by the far jump. (I *think* that's the explanation...) > Or is it, 1) setting cr0 bit 0 (PE) allows switching into protected mode, but doesn't do the switch 2) reloading CS with a selector instead of a segment actually does the switch into protected mode 3) the bit in the descriptor decides if it's 32-bit or 16-bit protected mode Rod Pemberton
From: Frank Kotler on 13 Aug 2007 05:58
Rod Pemberton wrote: > "Frank Kotler" <fbkotler(a)verizon.net> wrote in message > news:2%Uvi.1429$hK5.87(a)trndny02... > >>opexoc(a)gmail.com wrote: >> >>>Hello, >>> >>>look at some piece of code which is to be booted by BIOS at startup >>>and enter CPU into protected mode: >>> >>> 1[BITS 16] ; We need 16-bit intructions for Real mode >>> 2 >>> 3 [ORG 0x7C00] ; The BIOS loads the boot sector into memory >>>location 0x7C00 4 >>> 5 cli ; Disable interrupts, we want to >>>be alone >>> 6 >>> 7 xor ax, ax >>> 8 mov ds, ax ; Set DS-register to 0 - used by >>>lgdt >>> 9 >>> 10 lgdt [gdt_desc] ; Load the GDT descriptor >>> 11 >>> 12 mov eax, cr0 ; Copy the contents of CR0 into >>>EAX >>> 13 or eax, 1 ; Set bit 0 >>> 14 mov cr0, eax >>> 15 ; Copy the contents of EAX into >>>CR0 >>> 16 ;[BITS 32] >>> 17 jmp 08h:clear_pipe ; Jump to code segment, offset >>>clear_pipe >>> 18 [BITS 32] >>> 19 clear_pipe: >>> 20 mov ax, 10h ; Save data segment identifyer >>> 21 mov ds, ax >>> >>>In which location directive [BITS 32] should appear ( first or >>>second )? When I use first location then computer hang on and doesn't >>>execute properly code in clear_pipe. When I change first bit in cr0 >>>register then I enter into protected mode so I should use 32 bit >>>instruction. Despite of this fact only when I use second directive >>>[BITS 32] ( without first ) everything works ok. >> >>Well... the latter is correct. While you may be in "protected mode" >>after setting cr0, it's a bit in the descriptor loaded into cs that puts >>the CPU into 32-bit mode... so you're still in 16-bit mode until cs is >>loaded by the far jump. (I *think* that's the explanation...) >> > > > Or is it, > 1) setting cr0 bit 0 (PE) allows switching into protected mode, but doesn't > do the switch > 2) reloading CS with a selector instead of a segment actually does the > switch into protected mode > 3) the bit in the descriptor decides if it's 32-bit or 16-bit protected mode Yeah, that's probably a better way to put it. In any case, we're not in 32-bit mode until after the jump. Best, Frank |