diff options
Diffstat (limited to 'bootother.S')
| -rw-r--r-- | bootother.S | 50 |
1 files changed, 23 insertions, 27 deletions
diff --git a/bootother.S b/bootother.S index f5bd856..5e8348e 100644 --- a/bootother.S +++ b/bootother.S @@ -12,14 +12,19 @@ # mp.c causes each non-boot CPU in turn to jump to start. # mp.c puts the correct %esp in start-4, and the place to jump # to in start-8. +# +# This code is identical to bootasm.S except: +# - it does not need to enable A20 +# - it uses 0(start-4) for the %esp +# - it jumps to 0(start-8) instead of calling cmain -.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 PROT_MODE_CSEG, 0x8 # kernel code segment selector +.set PROT_MODE_DSEG, 0x10 # kernel data segment selector +.set CR0_PE_ON, 0x1 # protected mode enable flag .globl start start: - .code16 # This runs in real mode + .code16 # Assemble for 16-bit mode cli # Disable interrupts cld # String operations increment @@ -29,31 +34,21 @@ start: movw %ax,%es # -> Extra Segment movw %ax,%ss # -> Stack Segment - # Set up the stack pointer, growing downward from 0x7000-8. - movw $start-8,%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 # +//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 - # 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. + # Jump to next instruction, but in 32-bit code segment. + # Switches processor into 32-bit mode. ljmp $PROT_MODE_CSEG, $protcseg -# We are now 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 @@ -63,10 +58,11 @@ protcseg: movw %ax, %gs # -> GS movw %ax, %ss # -> SS: Stack Segment - movl start-8, %eax movl start-4, %esp + movl start-8, %eax jmp *%eax +# Bootstrap GDT .p2align 2 # force 4 byte alignment gdt: SEG_NULLASM # null seg |