From a650c606fecc7e3938345e2bc52050a9ea725e7a Mon Sep 17 00:00:00 2001 From: rsc <rsc> Date: Wed, 6 Sep 2006 17:04:06 +0000 Subject: spacing fixes: no tabs, 2-space indents (for rtm) --- bootasm.S | 139 +++++++++++++++++++++++++++++++++----------------------------- 1 file changed, 73 insertions(+), 66 deletions(-) (limited to 'bootasm.S') diff --git a/bootasm.S b/bootasm.S index f776b83..ed3a835 100644 --- a/bootasm.S +++ b/bootasm.S @@ -1,36 +1,37 @@ #include "asm.h" - -.set PROT_MODE_CSEG,0x8 # code segment selector + +.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 - +.set CR0_PE_ON,0x1 # protected mode enable flag + ################################################################################### -# ENTRY POINT +# ENTRY POINT # This code should be stored in the first sector of the hard disk. # After the BIOS initializes the hardware on startup or system reset, # it loads this code at physical address 0x7c00 - 0x7d00 (512 bytes). # Then the BIOS jumps to the beginning of it, address 0x7c00, # while running in 16-bit real-mode (8086 compatibility mode). # The Code Segment register (CS) is initially zero on entry. -# +# # This code switches into 32-bit protected mode so that all of # memory can accessed, then calls into C. ################################################################################### - -.globl start # Entry point -start: .code16 # This runs in real mode - cli # Disable interrupts - cld # String operations increment + +.globl start # Entry point +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 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 0x7c00. - movw $start,%sp # Stack Pointer - + # Set up the stack pointer, growing downward from 0x7c00. + movw $start,%sp # Stack Pointer + #### Enable A20: #### For fascinating historical reasons (related to the fact that #### the earliest 8086-based PCs could only address 1MB of physical memory @@ -38,59 +39,65 @@ start: .code16 # This runs in real mode #### physical address line 20 is tied to low when the machine boots. #### Obviously this a bit of a drag for us, especially when trying to #### address memory above 1MB. This code undoes this. - -seta20.1: inb $0x64,%al # Get status - testb $0x2,%al # Busy? - jnz seta20.1 # Yes - movb $0xd1,%al # Command: Write - outb %al,$0x64 # output port -seta20.2: inb $0x64,%al # Get status - testb $0x2,%al # Busy? - jnz seta20.2 # Yes - movb $0xdf,%al # Enable - outb %al,$0x60 # A20 + +seta20.1: + inb $0x64,%al # Get status + testb $0x2,%al # Busy? + jnz seta20.1 # Yes + movb $0xd1,%al # Command: Write + outb %al,$0x64 # output port -#### Switch from real to protected mode +seta20.2: + inb $0x64,%al # Get status + testb $0x2,%al # Busy? + jnz seta20.2 # Yes + movb $0xdf,%al # Enable + outb %al,$0x60 # A20 + +#### 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. - -real_to_prot: cli # Mandatory since we dont set up an IDT - 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 - +#### 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. + +real_to_prot: + cli # Mandatory since we dont set up an IDT + 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 - - call cmain # finish the boot load from C. - # cmain() should not return -spin: jmp spin # ..but in case it does, spin - -.p2align 2 # force 4 byte alignment +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 + + call cmain # finish the boot load from C. + # cmain() should not return +spin: + jmp spin # ..but in case it does, spin + + +.p2align 2 # force 4 byte alignment gdt: - SEG_NULLASM # null seg - SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg - SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg - + SEG_NULLASM # null seg + SEG_ASM(STA_X|STA_R, 0x0, 0xffffffff) # code seg + SEG_ASM(STA_W, 0x0, 0xffffffff) # data seg + gdtdesc: - .word 0x17 # sizeof(gdt) - 1 - .long gdt # address gdt + .word 0x17 # sizeof(gdt) - 1 + .long gdt # address gdt -- cgit v1.2.3