OS-Dev Notes 1
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