Babystep2: Difference between revisions
Revision as of 21:22, 23 January 2013
Babystep2: Writing a message using the BIOS
Writing a message using the BIOS
Quick review:
- Boot sector loaded by BIOS is 512 bytes
- The code in the boot sector of the disk is loaded by the BIOS at 0000:7c00
- Machine starts in Real Mode
- Be aware that the CPU is being interrupted unless you issue the CLI assembly command
Many (but not all) BIOS interrupts expect DS to be filled with a Real Mode segment value. This is why many BIOS interrupts won't work in protected mode. So if you want to use int 10h/ah=0eh to print to the screen, then you need to make sure that your seg:offset for the characters to print is correct.
In real mode, addresses are calculated as segment * 16 + offset. Since offset can be much larger than 16, there are many pairs of segment and offset that point to the same address. For instance, some say that the bootloader is is loaded at 0000:7C00, while others say 07C0:0000. This is in fact the same address: 16 * 0x0000 + 0x7C00 = 16 * 0x07C0 + 0x0000 = 0x7C00.
It doesn't matter if you use 0000:7c00 or 07c0:0000, but if you use ORG you need to be aware of what's happening. By default, the start of a raw binary is at offset 0, but if you need it you can change the offset to something different and make it work. For instance the following snippet accesses the variable msg with segment 0x7C0.
Asm Example:
; boot.asm movax,0x07c0 movds,ax movsi,msg ch_loop:lodsb oral,al; zero=end or str jzhang; get out movah,0x0E int0x10 jmpch_loop hang: jmphang msgdb'WelcometoMacintosh',13,10,0 times510-($-$$)db0 db0xAA db0x55
Here is the ORG version. This time, msg is accessed with segment 0. Note that you still need to tell DS what to be as it can hold any value.
[ORG0x7c00] xorax,ax; make it zero movds,ax movsi,msg ch_loop:lodsb oral,al; zero=end of string jzhang; get out movah,0x0E int0x10 jmpch_loop hang: jmphang msgdb'WelcometoMacintosh',13,10,0 times510-($-$$)db0 db0xAA db0x55
Procedures
To save on writing space, the typical 'procedures' are often separated from the code using CALL/RET like the following:
[ORG0x7c00] xorax,ax;make it zero movds,ax movsi,msg callbios_print hang: jmphang msgdb'WelcometoMacintosh',13,10,0 bios_print: lodsb oral,al;zero=end of str jzdone;get out movah,0x0E int0x10 jmpbios_print done: ret times510-($-$$)db0 db0xAA db0x55
For some inexplicable reason, loading SI then jumping to the procedure always bugged me. Fortunately for psychos like me NASM's macros let you pretend that you are passing a parameter (macro definitions has to go before it's being called).
%macroBiosPrint1 movsi,word%1 ch_loop:lodsb oral,al jzdone movah,0x0E int0x10 jmpch_loop done: %endmacro [ORG0x7c00] xorax,ax movds,ax BiosPrintmsg hang: jmphang msgdb'WelcometoMacintosh',13,10,0 times510-($-$$)db0 db0xAA db0x55
And in case your code is becoming long and unreadable, you can break it up into different files, then include the files at the beginning of you main code. Like so:
jmpmain %include"othercode.inc" main: ; ... rest of code here
Don't forget the jmp main at the start - otherwise some random other procedure will get called.