xv6-65oo2/bootother.S

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