Initial commit

boot/gdt.S

@@ -0,0 +1,39 @@
+.section .data
+gdt_start:  # don't remove the labels, they're needed to compute sizes and jumps
+    # the GDT starts with a null 8-byte
+    .long 0x0 # 4 byte
+    .long 0x0 # 4 byte
+
+# GDT for code segment. base = 0x00000000, length = 0xfffff
+# for flags, refer to os-dev.pdf document, page 36
+gdt_code: 
+    .word 0xffff # segment length, bits 0-15
+    .word 0x0    # segment base, bits 0-15
+    .byte 0x0    # segment base, bits 16-23
+    .byte 0b10011010 # flags (8 bits)
+    .byte 0b11001111 # flags (4 bits) + segment length, bits 16-19
+    .byte 0x0    # segment base, bits 24-31
+
+# GDT for data segment. base and length identical to code segment
+# some flags changed, again, refer to os-dev.pdf
+gdt_data: 
+    .word 0xffff
+    .word 0x0
+    .byte 0x0
+    .byte 0b10010010
+    .byte 0b11001111
+    .byte 0x0
+
+gdt_end: 
+
+# GDT descriptor
+gdt_descriptor:
+.long gdt_end-gdt_start-1
+.long gdt_start
+//MISMATCH: "    dw gdt_end - gdt_start - 1 "
+//MISMATCH: "    dd gdt_start "
+
+# define some constants for later use
+code_seg: .long gdt_code - gdt_start
+data_seg: .long gdt_data - gdt_start
+