Project 1: Assembly Language Tips

Tips

• Do not optimize prematurely (ie, before it works). It's fine to store all your variables on the heap instead of trying to use only registers (all the way until it must be faster).
• If it works in bochs but not on a real machine, then there is a bug in your program. In bochs, all memory is initially 0; this is not true on all computers. Registers, upon entering the bootloader, may not be the same. Even %dl (the boot device) may be different.
• The stack, upon entering your bootloader, might be good (the BIOS was just using it), but make sure you don't corrupt it unintentionally.
• The int instruction does, in fact, use the stack.
• rep mobsb is a quick and simply way (of many) to move data about.
• rep scasb can be used to implement strlen.

Examples

Please read the assembly language provided before reading this section. The file bootblock_examples.s also contains several x86 assembly language examples.

Loading a segment:offset
The project will require you to load a number into a segment register to setup the stack and data segments. The code segment register (CS) cannot be loaded directly, but instead only indirectly through a JMP type instruction. When loading a value into the stack segment register (SS), interrupts are disabled for the next instruction, thus allowing you to set the stack pointer (SP). As an example of setting up the segment registers for data, consider the following string copy:

# Setup the registers - see chapter 3 of Intel ISA reference volume 2
movw DATA_SEGMENT, %ax
movw %ax, %ds
movw OTHER_DATA_SEGMENT, %ax
movw %ax, %es
movw STRING_FROM_ADDRESS, %si
movw STRING_TO_ADDRESS, %di

# Move a byte from one string to the other - implictly DS:SI to ES:DI
movsb

# The values in %si and %di are automatically incremented/decremented based on the DF flag.

Display memory

For your design review you are required to implement routines that print to the screen. During booting, you can write directly to the screen by writing to the display RAM which is mapped starting at 0xb800:0000. Each location on the screen requires two bytes---one to specify the attribute (Use 0x07) and the second for the character itself. The text screen is 80x25 characters. So, to write to i-th row and j-th column, you write the 2 bytes starting at offset ((i-1)*80+(j-1))*2.

So, the following code sequence writes the character 'K' (ascii 0x4b) to the top left corner of the screen.
movw $0xb800,%bx

movw %bx,%es

movw $0x074b,%es:(0x0)

This code sequence is very useful for debugging.

Useful BIOS Features

BIOS Int 0x13 Function 2 (From Undocumented PC)

Called with:
ah = 2
al = number of sectors to read,
ch = cylinder number, (lowest 8 bits of the 10-bit cylinder number, 0 based)
cl, bits 6&7 = cylinder number bits 8 and 9.
bits 0-5 = starting sector number, 1 to 63

dh = starting head number, 0 to 255
dl = drive number. Use the value of dl that you stored at the beginning of the bootloader.
es:bx = pointer where to place information read from diskette

Returns:
ah = return status (0 if successful)
al = burst error length if ah = 11; undefined on most BIOSes for other ah return status values.
carry = 0 successful, = 1 if error occurred

BIOS Int 0x10 Function 0x0e (From Undocumented PC)

Called with:
ah = 0x0e
al = character to write
bh = active page number (Use 0x00)
bl = foreground color (graphics mode only) (Use 0x02)

Returns:
character displayed