.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
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
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
# 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
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
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
projects / cs3210-lab1.git / commitdiff