diff options
Diffstat (limited to 'bootasm.S')
| -rw-r--r-- | bootasm.S | 46 |
1 files changed, 23 insertions, 23 deletions
@@ -4,7 +4,7 @@ .set PROT_MODE_DSEG,0x10 # data segment selector .set CR0_PE_ON,0x1 # protected mode enable flag -################################################################################### +######################################################################### # 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, @@ -15,7 +15,7 @@ # # This code switches into 32-bit protected mode so that all of # memory can accessed, then calls into C. -################################################################################### +######################################################################### .globl start # Entry point start: @@ -32,13 +32,13 @@ 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 -#### and subsequent 80286-based PCs wanted to retain maximum compatibility), -#### 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. + # Enable A20: + # For fascinating historical reasons (related to the fact that + # the earliest 8086-based PCs could only address 1MB of physical + # memory and subsequent 80286-based PCs wanted to retain maximum + # compatibility), 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 @@ -54,22 +54,22 @@ seta20.2: 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. + # 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 # + 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. |