head.s

用于汇编领域的,运用于OS的MAIN函数.基于硬件基础的源代码

.globl	startup
.globl  GDT

.equ	GDT_ENTRIES,4
.equ	GDT_ENTRY_SZ,8
.equ	SEG1,1<<3   # kernel cs
.equ	SEG2,2<<3   # kernel ds
.equ    SEG3,3<<3   # tss

startup:
        cli
	lgdt	GDT_pointer     # redo it here
        call    start_kernel
# Maybe NEVER REACHED...

##########################################
# GDT & IDT data
##########################################
.align 8
.word	0,0,0,0
GDT:
	# Descriptor 0 : NULL segment
#.word	0
#.word	0
#.word	0
#.word	0
        .quad 0x0000000000000000

	# Descriptor 1 : Kernel code segment
#.word	0xFFFF			# bytes 0 and 1 of segment size
#.word	0x0000			# bytes 0 and 1 of segment base address
#.byte	0x00			# byte 2 of segment base address
#.byte	0x9A			# present, DPL=0, code exec-read
#.byte	0xCF			# 4kb graanularity, 32bit code
#.byte	0x00			# byte 3 of segment base address
	.quad 0x00cf9a000000ffff

	# Descriptor 2 : Kernel data and stack segment
#.word	0xFFFF
#.word	0x0000
#.byte	0x00
#.byte	0x92			# present, DPL=0, data read/write
#.byte	0xCF
#.byte	0x00
	.quad 0x00cf92000000ffff

        # Descriptor 3 : tss. will be filled in later
#.word   0x00
#.word   0x00
#.word   0x00
#.word   0x00
	.quad 0x000000000000

GDT_pointer:
	.word	GDT_ENTRIES * GDT_ENTRY_SZ	# limit
#.word	0x9A00 + GDT		# base address
	.long   GDT
	.word	0

	.word	0			# padding