78 lines
2.8 KiB
ArmAsm
78 lines
2.8 KiB
ArmAsm
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#include "asm.h"
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/*
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* Start an Application Processor. This must be placed on a 4KB boundary
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* somewhere in the 1st MB of conventional memory (APBOOTSTRAP). However,
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* due to some shortcuts below it's restricted further to within the 1st
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* 64KB. The AP starts in real-mode, with
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* CS selector set to the startup memory address/16;
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* CS base set to startup memory address;
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* CS limit set to 64KB;
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* CPL and IP set to 0.
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*
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* Credit: Cliff Frey
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*/
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.set PROT_MODE_CSEG,0x8 # code segment selector
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.set PROT_MODE_DSEG,0x10 # data segment selector
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.set CR0_PE_ON,0x1 # protected mode enable flag
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.globl start
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start: .code16 # This runs in real mode
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cli # Disable interrupts
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cld # String operations increment
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# Set up the important data segment registers (DS, ES, SS).
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xorw %ax,%ax # Segment number zero
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movw %ax,%ds # -> Data Segment
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movw %ax,%es # -> Extra Segment
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movw %ax,%ss # -> Stack Segment
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# Set up the stack pointer, growing downward from 0x7000.
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movw $start,%sp # Stack Pointer
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#### Switch from real to protected mode
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#### The descriptors in our GDT allow all physical memory to be accessed.
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#### Furthermore, the descriptors have base addresses of 0, so that the
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#### segment translation is a NOP, ie. virtual addresses are identical to
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#### their physical addresses. With this setup, immediately after
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#### enabling protected mode it will still appear to this code
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#### that it is running directly on physical memory with no translation.
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#### This initial NOP-translation setup is required by the processor
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#### to ensure that the transition to protected mode occurs smoothly.
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lgdt gdtdesc # load GDT -- mandatory in protected mode
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movl %cr0, %eax # turn on protected mode
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orl $CR0_PE_ON, %eax #
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movl %eax, %cr0 #
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### CPU magic: jump to relocation, flush prefetch queue, and reload %cs
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### Has the effect of just jmp to the next instruction, but simultaneous
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### loads CS with $PROT_MODE_CSEG.
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ljmp $PROT_MODE_CSEG, $protcseg
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#### we are in 32-bit protected mode (hence the .code32)
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.code32
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protcseg:
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# Set up the protected-mode data segment registers
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movw $PROT_MODE_DSEG, %ax # Our data segment selector
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movw %ax, %ds # -> DS: Data Segment
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movw %ax, %es # -> ES: Extra Segment
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movw %ax, %fs # -> FS
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movw %ax, %gs # -> GS
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movw %ax, %ss # -> SS: Stack Segment
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# XXX hack
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movl 0x10018, %eax # elfhdr->entry (left over in scratch space)
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# subl $KERNBASE, %eax
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jmp *%eax # this jumps to _start in kern/entry.S
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.p2align 2 # force 4 byte alignment
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gdt:
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SEG_NULL # null seg
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SEG(STA_X|STA_R, 0x0, 0xffffffff) # code seg
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SEG(STA_W, 0x0, 0xffffffff) # data seg
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gdtdesc:
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.word 0x17 # sizeof(gdt) - 1
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.long gdt # address gdt
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