OS-Dev Notes 1

Written at 01 Oct, 2021

OS-DEV notes #1

8086 Segment Registers:

To access memory adresses more than 16 bit (65535) we use 'Segment Registers'
It is always appreciated to remove interrupts (cli), reset segment registers, then set interrupts (sli) before bootloader starts.

CS => Code Segment
SS => Stack Segment
DS => Data Segment
ES => Extra Segment

Final memory address is calculated as:
final address = Segment Register * 16 + offset

In the example below, lodsb knows that, address it must read is 0x7c * 16 + 0x1f, as lodsb uses Data Segment

org 0
mov ax, 0x7c
mov ds, ax
mov si, 0x1f
lodsb           ; LODSB uses Data Segment for segmentation

The table below demonstrates LOD__ command usage:

Code	Mnemonic	Description
AC	LODS m8	Load byte at address DS:(E)SI into AL
AD	LODS m16	Load word at address DS:(E)SI into AX
AD	LODS m32	Load double-word at address DS:(E)SI into EAX
AC	LODSB	Load byte at address DS:(E)SI into AL
AD	LODSW	Load word at address DS:(E)SI into AX
AD	LODSD	Load double-word at address DS:(E)SI into EAX

Stack Segmentation example:
At the code below, when we push 0xffff to stack, stack pointer is decremented by 2 (bc, we use 16 bit) and sp == 0x7bfe.
Then, memory at adress 0x7bfe is set to 0xffff.

mov ax, 0x00    ; Set ax to 0x00
mov ss, ax      ; Move value in ax to 'stack segment'
mov ax, 0x7c00  ; Add ax to 0x7c00
mov sp, ax      ; Move AX to 'stack pointer'

push 0xffff     ; Pushes 0xffff to the stack