diff options
Diffstat (limited to 'bootasm.S')
| -rw-r--r-- | bootasm.S | 30 |
1 files changed, 15 insertions, 15 deletions
@@ -1,23 +1,23 @@ #include "asm.h" - + .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 - + ################################################################################### -# 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 + +.globl start # Entry point start: .code16 # This runs in real mode cli # Disable interrupts @@ -31,7 +31,7 @@ start: # 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 @@ -39,7 +39,7 @@ start: #### 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? @@ -54,7 +54,7 @@ seta20.2: movb $0xdf,%al # Enable outb %al,$0x60 # A20 -#### Switch from real to protected mode +#### 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 @@ -63,21 +63,21 @@ seta20.2: #### 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 # + 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: +protcseg: # Set up the protected-mode data segment registers movw $PROT_MODE_DSEG, %ax # Our data segment selector movw %ax, %ds # -> DS: Data Segment @@ -97,7 +97,7 @@ gdt: 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 |