Real Mode Assembly: Print Char to Screen without INT Instruction on Boot
The following site "Writing Boot Sector Code" provides a sample of code that prints 'A' to the screen when the system boots. From what I have been reading don't you have to use INT opcode to get BIOS to do certain things? How does the code below, from the site referenced above work without using interrupts? What portion of code actually tells the hardware to print 'A' to the screen?
Code in question:
.code16
.section .text
.globl _start
_start:
mov $0xb800, %ax
mov %ax, %ds
movb $'A', 0
movb $0x1e, 1
idle:
jmp idle
APPENDING TO ORIGINAL QUESTION
If I use the following code does the BIOS call write to the text buffer for me? The buffer starting at address 0xb800?
# Author: Matthew Hoggan
# Date Created: Tuesday, Mar 6, 2012
.code16 # Tell assembler to work in 16 bit mode (directive)
.section .text
.globl _start # Help linker find start of program
_start:
movb $0x0e, %ah # Function to print a character to the screen
movb $0x00, %bh # Indicate the page number
movb $0x07, %bl # Text attribute
mov $'A', %al # Move data into low nibble
int $0x10 # Video Service Request to Bios
_hang:
jmp _hang
.end
-
It does. Though, the buffer location really depends on the graphics chip. On monochrome adapters you'd use segment 0xb000 instead of 0xb800. – Alexey Frunze Mar 12 '12 at 6:29
Direct answer to your question: The line "movb $'A', 0" effectively completes the print to the screen (and the following line, "movb $0x1e, 1" specifies what color it should be).
Longer answer: The video hardware draws the screen based on the contents of memory. When in text mode, the video hardware starts drawing based on memory segment 0xB800. Whatever is at byte 0 defines the character to be drawn at the first text cell on the screen. The next byte defines the attributes (foreground color, background color, and blink status). This pattern repeats (char - attr - char - attr) throughout the entire screen.
So, technically, my direct answer wasn't true. The 2 'movb' statements simply stage the letter 'A' to be printed. 'A' is not printed until the next time hardware refreshes the display based on the memory.
-
Two questions, the hardware will refresh at its own refresh rate? Is that based on CPU cycles? If I wanted to read more up on this do you have any good references? – Matthew Hoggan Mar 11 '12 at 7:40
-
Yeah, the hardware does its own thing. When I think of these text modes, I'm thinking of MS-DOS from 30 years ago. Monitors were running at 50 or 60 Hz then (PAL or NTSC, respectively). So that was 50 or 60 refreshes per second. Most video hardware allows software to poll a bit in order to learn if a vertical refresh is occurring. This is useful for preventing graphical flicker but it's not usually a huge concern for text mode. As for references, I guess the Wikipedia article for text mode would be a good start: en.wikipedia.org/wiki/Text_mode – Multimedia Mike Mar 11 '12 at 7:58
