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> OS Construction > About protected mode and memory |
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cod3b453 15 Sep 2012, 15:06
Normally protected mode is 32 or 64 bit; EMS and XMS are 16bit real mode extensions. For simplicity, I'd recommend you go for 32bit protected mode, since you can use memory without the need to set up page tables. This requires you enable A20, create a valid GDT and perform the switch with correct descriptors and stack setup. After that it should be easy to add other functionality as you go.
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15 Sep 2012, 15:06 |
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A$M 15 Sep 2012, 17:52
Okay, but how can I do this? Please write a sample code... And how can I access RAM memory in a large scale.
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15 Sep 2012, 17:52 |
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cod3b453 15 Sep 2012, 18:39
Below is a cut down version of an old snapshot from my 32bit OS boot loader.
INCLUDES: Code: ; ./hardware/gdt.ash GDT_COUNT equ 0x2000 GDT_SIZE equ (sizeof.GDT_ENTRY * GDT_COUNT) ; GDT Selector Rings GDT_RPL_RING0 equ 0x0000 GDT_RPL_RING1 equ 0x0001 GDT_RPL_RING2 equ 0x0002 GDT_RPL_RING3 equ 0x0003 GDT_RPL_MASK equ 0x0003 ; GDTENTRY.Access GDT_PRESENT equ 0x80 GDT_RING0 equ 0x00 GDT_RING1 equ 0x20 GDT_RING2 equ 0x40 GDT_RING3 equ 0x60 GDT_CODE equ 0x10 GDT_CODE_EXECUTE equ 0x08 GDT_CODE_READ equ 0x02 GDT_DATA equ 0x10 GDT_DATA_WRITE equ 0x02 GDT_DATA_READ equ 0x00 GDT_STACK equ 0x10 GDT_STACK_WRITE equ 0x02 GDT_STACK_READ equ 0x00 GDT_STACK_EXPANDDOWN equ 0x04 GDT_STACK_EXPANDUP equ 0x00 GDT_SYSTEM equ 0x00 ; GDTENTRY.Flags GDT_GRANULARITY_BYTES equ 0x00 GDT_GRANULARITY_PAGES equ 0x80 GDT_16BIT equ 0x00 GDT_32BIT equ 0x40 GDT_USER equ 0x10 GDT_ACCESS_MASK equ (GDT_PRESENT or GDT_RING3 or GDT_DATA or GDT_CODE_EXECUTE or GDT_STACK_EXPANDDOWN or GDT_DATA_WRITE) GDT_FLAGS_MASK equ (GDT_GRANULARITY_PAGES or GDT_32BIT or GDT_USER) GDT_LIMITHI_MASK equ 0x0F struc GDTENTRY b,l,f,a { .LimitLo dw (l and 0xFFFF) .BaseLo dw (b and 0xFFFF) .BaseMi db ((b shr 16) and 0xFF) .Access db (a and GDT_ACCESS_MASK) .Flags_LimitHi db ((f and GDT_FLAGS_MASK) or ((l shr 16) and GDT_LIMITHI_MASK)) .BaseHi db (b shr 24) } struc GDTR32ENTRY b,e { .Size dw (e - b - 1) .Address dd (b) } ;==== ; ./hardware/i804x.ash ; Port Designations i804x_PORT_READ equ 0x0060 ; R i804x_PORT_WRITE equ 0x0060 ; W i804x_PORT_STATUS equ 0x0064 ; R i804x_PORT_COMMAND equ 0x0064 ; RW ; Input Flags i804X_INPUT_KEYBOARDENABLED equ 0x80 i804X_INPUT_COLOUR equ 0x40 i804x_INPUT_MANUFACTURER equ 0x20 i804x_INPUT_EXTERNALRAM equ 0x10 i804x_INPUT_MOUSEDATA equ 0x02 i804x_INPUT_KEYBOARDDATA equ 0x01 ; Output Flags i804x_OUTPUT_KEYBOARDDATA equ 0x80 i804x_OUTPUT_KEYBOARDCLOCK equ 0x40 i804x_OUTPUT_IRQ12 equ 0x20 i804x_OUTPUT_IRQ01 equ 0x10 i804x_OUTPUT_MOUSECLOCK equ 0x08 i804x_OUTPUT_MOUSEDATA equ 0x04 i804x_OUTPUT_A20 equ 0x02 i804x_OUTPUT_SYSTEM equ 0x01 ; Test Flags i804x_TEST_MOUSECLOCK equ 0x02 i804x_TEST_KEYBOARDCLOCK equ 0x01 ; Status Register (Read i804x_PORT_STATUS) i804x_STATUS_PARITYERROR equ 0x80 i804x_STATUS_TIMEOUT equ 0x40 i804x_STATUS_MOUSEOUPUTBUFFERFULL equ 0x20 i804x_STATUS_INHIBIT equ 0x10 i804x_STATUS_A2 equ 0x08 i804x_STATUS_SYSTEM equ 0x04 i804x_STATUS_INPUTBUFFERFULL equ 0x02 i804x_STATUS_OUTPUTBUFFERFULL equ 0x01 ; Command Register (Write i804x_PORT_COMMAND) i804x_COMMAND_TRANSLATE equ 0x40 i804x_COMMAND_DISABLEMOUSE equ 0x20 i804x_COMMAND_DISABLEKEYBOARD equ 0x10 i804x_COMMAND_SYSTEM equ 0x04 i804x_COMMAND_IRQ12 equ 0x02 i804x_COMMAND_IRQ01 equ 0x01 ; Command List (Write i804x_PORT_COMMAND) i804x_COMMAND_READCOMMAND equ 0x20 ; ? i804x_COMMAND_WRITECOMMAND equ 0x60 ; ? i804x_COMMAND_READINPUT equ 0xC0 ; Read i804x_INPUT_xxxx from i804x_PORT_READ i804x_INHIBIT equ 0x00 i804x_ENABLE equ 0x80 i804x_MONO equ 0x40 i804x_COLOR equ 0x00 i804x_JUMPER equ 0x20 i804x_RAM equ 0x10 i804x_COMMAND_WRITEINPUTLO equ 0xC1 ; PS2 only; Write i804x_INPUT_xxxx to i804x_PORT_WRITE i804x_COMMAND_WRITEINPUTHI equ 0xC2 ; PS2 only; Write i804x_INPUT_xxxxto i804x_PORT_WRITE i804x_COMMAND_READOUTPUT equ 0xD0 ; Read i804x_OUTPUT_xxxx from i804x_PORT_READ i804x_COMMAND_WRITEOUTPUT equ 0xD1 ; Write i804x_OUTPUT_xxxx to i804x_PORT_WRITE i804x_KEYBOARDDATA equ 0x80 i804x_KEYBOARDCLOCK equ 0x40 i804x_INPUTBUFFEREMPTY equ 0x20 i804x_OUTPUTBUFFERFULL equ 0x10 i804x_A20 equ 0x02 i804x_RESET equ 0x01 i804x_COMMAND_WRITEKEYBOARD equ 0xD2 ; Emulates keyboard signal i804x_COMMAND_WRITEMOUSE equ 0xD4 ; Emulates mouse signal i804x_COMMAND_RESET equ 0xFF ; ? i804x_ACK equ 0xFA Code: ;==== ; ./boot/boot.asm use16 org 0x7C00 ;==== ; ./loader/loader.asm call a20.enable cli lgdt [gdtr32] mov eax,cr0 or eax,1 mov cr0,eax jmp pword 0x0008:pmode a20: .enable: cli call a20.enable_i804x call a20.test sti ret .enable_i804x: push ax mov al,i804x_COMMAND_READOUTPUT out i804x_PORT_COMMAND,al call .enable_i804x_wait in al,i804x_PORT_READ or al,(i804x_OUTPUT_IRQ01 or i804x_OUTPUT_A20) mov ah,al call .enable_i804x_wait mov al,i804x_COMMAND_WRITEOUTPUT out i804x_PORT_COMMAND,al call .enable_i804x_wait mov al,ah out i804x_PORT_WRITE,al call .enable_i804x_wait mov al,i804x_COMMAND_RESET out i804x_PORT_COMMAND,al call .enable_i804x_wait pop ax ret .enable_i804x_wait: push ax @@: in al,i804x_PORT_STATUS and al,i804x_STATUS_INPUTBUFFERFULL jnz @b pop ax ret .test: push fs gs xor ax,ax mov fs,ax not ax mov gs,ax mov al,byte [fs:0x0000] mov ah,al not al xchg al,byte [gs:0x0010] clc cmp ah,byte [fs:0x0000] mov byte [gs:0x0010],al je @f stc @@: pop gs fs ret use32 align 16 pmode: mov eax,0x0010 mov ds,ax mov es,ax mov fs,ax mov gs,ax mov ss,ax mov esp,0x00100000 @@: hlt jmp @b gdtr32 GDTR32ENTRY gdt32.start,gdt32.end gdt32: .start: .null GDTENTRY 0x00000000,0x00000000,0x00,0x00 .code GDTENTRY 0x00000000,0x000FFFFF,(GDT_GRANULARITY_PAGES or GDT_32BIT),(GDT_PRESENT or GDT_RING0 or GDT_CODE or GDT_CODE_READ or GDT_CODE_EXECUTE) .data GDTENTRY 0x00000000,0x000FFFFF,(GDT_GRANULARITY_PAGES or GDT_32BIT),(GDT_PRESENT or GDT_RING0 or GDT_DATA or GDT_DATA_READ or GDT_DATA_WRITE) .end: dw 0 .signature dw 0xAA55 ;==== Last edited by cod3b453 on 17 Sep 2012, 17:10; edited 3 times in total |
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15 Sep 2012, 18:39 |
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ACP 15 Sep 2012, 18:39
A$M wrote: Okay, but how can I do this? Please write a sample code... And how can I access RAM memory in a large scale. This sub-forum is full of examples - just enter pm or protected mode and press "search: button. Keep in mind that in some cases you can get away with flat mode instead of protected mode. This applies to cases when you don't need memory protection and/or paging but would like to escape data segmentation. A variation on flat mode allowing to have 32bit code adders space has been discussed here too - just search for unreal mode. However keep in mind that if you don't know nothing about how memory management works, what a descriptor or IDT is, any sample code will be more like black magic to you than something useful. You first need to graps some theory to understand example code. |
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15 Sep 2012, 18:39 |
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A$M 15 Sep 2012, 19:08
You can type a simpler example? I did not understand much.
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15 Sep 2012, 19:08 |
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ACP 16 Sep 2012, 10:21
A$M wrote: You can type a simpler example? I did not understand much. You haven't read my post have you? First learn what PM is, than start to read code. |
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16 Sep 2012, 10:21 |
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A$M 17 Sep 2012, 17:36
I found a PDF document that tells you how to enter the protected mode and so I wrote this code:
Code: org 7c00h use16 start: xor ax, ax mov ds, ax mov es, ax mov fs, ax mov gs, ax mov sp, 7c00h mov ah, 24h int 15h cli lgdt [gdtr] mov eax, cr0 or eax, 1 mov cr0, eax jmp 10h:protected use32 protected: sti ; Continue code here... gdt dw 0x0000, 0x0000, 0x0000, 0x0000 sys_data dw 0xffff, 0x0000, 0x9200, 0x00cf sys_code dw 0xffff, 0x0000, 0x9800, 0x00cf gdt_end: gdtr dw gdt_end-gdt-1 dd gdt times 510- ($-start) db 0 dw 0xaa55 I understood everything. Except the GDT. Can you explain me? What are these values? What is GDT? This code is right? (In theory) Last edited by A$M on 17 Sep 2012, 17:42; edited 1 time in total |
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17 Sep 2012, 17:36 |
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cod3b453 17 Sep 2012, 17:42
Tidied my previous post a little.
It's a good idea to get copies of the Intel and AMD manuals; these also have their own examples and explanations. The A20 code is derived from stuff I read online a long time ago, so I can't give you a concrete reference for that. You shouldn't do sti until you have loaded valid selectors into ds,es and ss and loader a valid IDTR/IDT. |
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17 Sep 2012, 17:42 |
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A$M 17 Sep 2012, 17:47
But this code works?
I will try soon. But can you try for me? |
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17 Sep 2012, 17:47 |
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A$M 17 Sep 2012, 17:50
Okay, but what is IDTR/IDT?
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17 Sep 2012, 17:50 |
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f0dder 17 Sep 2012, 19:01
A$M wrote: Okay, but what is IDTR/IDT? _________________  - carpe noctem |
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17 Sep 2012, 19:01 |
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A$M 17 Sep 2012, 19:17
Oh! IDT = Interrupt Descriptor Table... Sorry! I did not understand the acronym... But now I understand. As I'm distracted...
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17 Sep 2012, 19:17 |
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A$M 17 Sep 2012, 19:57
What values should I put in the GDT? What is it?
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17 Sep 2012, 19:57 |
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A$M 19 Sep 2012, 23:56
I discovered what I did not know about the GDT. I "googled". That left little doubt. But if someone wants to add information or a question about Protected Mode or about memory management, feel free.
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19 Sep 2012, 23:56 |
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f0dder 20 Sep 2012, 19:25
There's already a lot of information on these things around - both beginner and more advanced stuff, both on this messageboard and other places (osdev.org isn't the only place). You really should familiarize yourself with the basics before asking a ton of questions that already have answers.
Of course if you spend some time researching but have trouble understanding parts of it, or can't get code working, don't hesitate to ask - but your posts seem kinda of the "OK-I've-done-this-tell-me-what-the-next-step-is" type  |
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20 Sep 2012, 19:25 |
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A$M 21 Sep 2012, 13:36
f0dder wrote: There's already a lot of information on these things around - both beginner and more advanced stuff, both on this messageboard and other places (osdev.org isn't the only place). You really should familiarize yourself with the basics before asking a ton of questions that already have answers. If you say... Who am I to contradict?  |
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21 Sep 2012, 13:36 |
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