lessons 8, 9, 10, entering 32-bit mode

09-32bit-gdt/32bit-gdt.asm

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+gdt_start: ; don't remove the labels, they're needed to compute sizes and jumps
+    ; the GDT starts with a null 8-byte
+    dd 0x0 ; 4 byte
+    dd 0x0 ; 4 byte
+
+; GDT for code segment. base = 0x00000000, length = 0xfffff
+; for flags, refer to os-dev.pdf document, page 36
+gdt_code: 
+    dw 0xffff    ; segment length, bits 0-15
+    dw 0x0       ; segment base, bits 0-15
+    db 0x0       ; segment base, bits 16-23
+    db 10011010b ; flags (8 bits)
+    db 11001111b ; flags (4 bits) + segment length, bits 16-19
+    db 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:
+    dw 0xffff
+    dw 0x0
+    db 0x0
+    db 10010010b
+    db 11001111b
+    db 0x0
+
+gdt_end:
+
+; GDT descriptor
+gdt_descriptor:
+    dw gdt_end - gdt_start - 1 ; size (16 bit), always one less of its true size
+    dd gdt_start ; address (32 bit)
+
+; define some constants for later use
+CODE_SEG equ gdt_code - gdt_start
+DATA_SEG equ gdt_data - gdt_start

09-32bit-gdt/README.md

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+*Concepts you may want to Google beforehand: GDT*
+
+**Goals: program the GDT**
+
+Remember segmentation from lesson 6? The offset was left shifted
+to address an extra level of indirection.
+
+In 32-bit mode, segmentation works differently. Now, the offset becomes an
+index to a segment descriptor (SD) in the GDT. This descriptor defines
+the base address (32 bits), the size (20 bits) and some flags, like
+readonly, permissions, etc. To add confusion, the data structures are split,
+so open the os-dev.pdf file and check out the figure on page 34 or the 
+Wikipedia page for the GDT.
+
+The easiest way to program the GDT is to define two segments, one for code
+and another for data. These can overlap which means there is no memory protection,
+but it's good enough to boot, we'll fix this later with a higher language.
+
+As a curiosity, the first GDT entry must be `0x00` to make sure that the
+programmer didn't make any mistakes managing addresses.
+
+Furthermore, since the CPU needs to know how long the GDT is, we'll use
+a meta structure called the "GDT descriptor" with the size (16b) and address
+(32b) of our actual GDT.
+
+Let's directly jump to the GDT code in assembly. Again, to understand
+all the segment flags, refer to the os-dev.pdf document. The theory for
+this lesson is quite complex.
+
+In the next lesson we will make the switch to 32-bit protected mode
+and test our code from these lessons.