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#1 |
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The Supreme Ruler
![]() Join Date: May 2004
Location: Houston
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; This is a comment [ORG 0x7C00];This just tells the program where it is in the memory. Not important [BITS 16];Not important too. jmp start ; Jump over BIOS parameter block start:;The label for the start of the actual program push cs;Put cs onto the stack pop ds;Take it out and put it into ds mov si,Print_loading ;Print loading message call printstring ;Call is like jump, but it goes back. Like a function ;The complicated bit: Loads the next program mov ah,02h ;When ah=, int13 reads a disk sector mov al,4 ;Al is how many sectors to read mov ch,0 ;The track to read from mov cl,2 ;Sector Id mov dh,0 ;Head mov dl,0 ;Drive (0 is floppy) mov bx,0x1000 ;Es and Bx put together are where to load the program too (see jmp 0x1000:0x00) mov es,bx mov bx,0x00 int 13h ;Int 13 is all functions for disks mov si,putdot ;Print a ".". call printstring jmp 0x1000:0x00 ;Run Bootinit from stack. printstring: ;Print string routine. mov ah,0eh ;Mov ah into 0, so int 10 prints stringloop: ;The following code loads each seperate charcter so it can be printed lodsb cmp al,00 ;If al =0, then the string has all been loaded je endstring int 10h ;When int 10 is called, and ah=, it prints jmp stringloop endstring: ret ;Ret returns putdot db '.',0 Print_loading db 13,10,'Loading Easy OS v0.01a...',0 times 425 db 0;wastes 425 bytes on purpose, so the sector is full (The program must be 512 bytes long) I was looking at someone elses code, and right above printstring there is a jmp 0x1000:0x00. Why is it jumping there? Here's the site: http://www.groovyweb.uklinux.net/index.php...your%20own%20os
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"Every gun that is made, every warship launched, every rocket signifies, in the final sense, a theft from those who hunger and are not fed, from those who are cold and are not clothed. The world in arms is not spending money alone. It is spending the sweat of its laborers, the genius of its scientists, the hopes of its children." - Dwight D. Eisenhower |
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#2 |
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Professional Programmer
Join Date: Nov 2004
Posts: 250
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>Why is it jumping there?
The key is here (especially the comment): mov bx,0x1000 ;Es and Bx put together are where to load the program too (see jmp 0x1000:0x00) mov es,bx mov bx,0x00 |
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#3 |
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The Supreme Ruler
![]() Join Date: May 2004
Location: Houston
Posts: 1,476
Rep Power: 6
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Ahh. I feel dumb now. Don't know how I missed the comment. Thanks for the help. That made it clearer.
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"Every gun that is made, every warship launched, every rocket signifies, in the final sense, a theft from those who hunger and are not fed, from those who are cold and are not clothed. The world in arms is not spending money alone. It is spending the sweat of its laborers, the genius of its scientists, the hopes of its children." - Dwight D. Eisenhower |
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#4 |
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Expert Programmer
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Back in the old say, before windows when writing a program the first executable location in memory was segment 0x1000 offset 0x0000 is I recall correctly, and I believe that is what the program comment was explaining.
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Clifford Matthew Roche <geek@cliffordroche.com> Web Hosting: http://www.crd-hosting.com Consulting: http://www.crdev-consulting.com |
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