diff options
Diffstat (limited to 'bootasm.S')
| -rw-r--r-- | bootasm.S | 56 |
1 files changed, 25 insertions, 31 deletions
@@ -9,16 +9,17 @@ .set PROT_MODE_DSEG, 0x10 # kernel data segment selector .set CR0_PE_ON, 0x1 # protected mode enable flag -.globl start # Entry point +.globl start start: -.code16 # This runs in real mode - cli # Disable interrupts + .code16 # Assemble for 16-bit 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 + xorw %ax,%ax # Segment number zero + movw %ax,%ds # -> Data Segment + movw %ax,%es # -> Extra Segment + movw %ax,%ss # -> Stack Segment # Enable A20: # For backwards compatibility with the earliest PCs, physical @@ -40,28 +41,21 @@ seta20.2: movb $0xdf,%al # 0xdf -> port 0x60 outb %al,$0x60 -# 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. +//PAGEBREAK! + # Switch from real to protected mode, using a bootstrap GDT + # and segment translation that makes virtual addresses + # identical to their physical addresses, so that the + # effective memory map does not change during the switch. + lgdt gdtdesc + movl %cr0, %eax + orl $CR0_PE_ON, %eax + movl %eax, %cr0 + + # Jump to next instruction, but in 32-bit code segment. + # Switches processor into 32-bit mode. ljmp $PROT_MODE_CSEG, $protcseg -#### we are in 32-bit protected mode (hence the .code32) -.code32 + .code32 # Assemble for 32-bit mode protcseg: # Set up the protected-mode data segment registers movw $PROT_MODE_DSEG, %ax # Our data segment selector @@ -71,13 +65,13 @@ protcseg: movw %ax, %gs # -> GS movw %ax, %ss # -> SS: Stack Segment - # Set up the stack pointer, growing downward from 0x7c00. + # Set up the stack pointer and call into C. movl $start, %esp + call cmain - call cmain # finish the boot load from C. - # cmain() should not return + # If cmain returns (it shouldn't), loop. spin: - jmp spin # ..but in case it does, spin + jmp spin # Bootstrap GDT .p2align 2 # force 4 byte alignment |