Writing a Simple Operating System — from Scratch

CHAPTER 3. BOOT SECTOR PROGRAMMING (IN 16-BIT REAL

MODE) 18

;

; A simple boot sector program that demonstrates the stack.

;

mov ah, 0x0e ; int 10/ah = 0eh -> scrolling teletype BIOS routine

mov bp, 0x8000 ; Set the base of the stack a little above where BIOS

mov sp, bp ; loads our boot sector - so it won’t overwrite us.

push ’A’ ; Push some characters on the stack for later

push ’B’ ; retreival. Note , these are pushed on as

push ’C’ ; 16-bit values , so the most significant byte

; will be added by our assembler as 0x00.

pop bx ; Note , we can only pop 16-bits , so pop to bx

mov al, bl ; then copy bl (i.e. 8-bit char) to al

int 0x10 ; print(al)

pop bx ; Pop the next value

mov al, bl

int 0x10 ; print(al)

mov al, [0 x7ffe] ; To prove our stack grows downwards from bp ,

; fetch the char at 0x8000 - 0x2 (i.e. 16-bits)

int 0x10 ; print(al)

jmp $ ; Jump forever.

; Padding and magic BIOS number.

times 510-($-$$) db 0

dw 0xaa55

Figure 3.6: Manipulation of the stack, usingpushandpop

Conditional jumps are achieved in assembly language by first running a comparison
instruction, then by issuing a specific conditional jump instruction.

cmp ax, 4 ; compare the value in ax to 4

je then_block ; jump to then_block if they were equal

mov bx, 45 ; otherwise , execute this code

jmp the_end ; important: jump over the ’then’ block ,

; so we don’t also execute that code.

then_block:

mov bx, 23

the_end:

In a language such as C or Java, this would look like this:

if(ax == 4) {

bx = 23;

} else {

bx = 45;

}