I have been reading and following a tutorial on writing an operating system from scratch by Nick Blundell which can be found at https://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf
I have successfully written a boot loader that can call C code, and so I have started writing my kernel in C. I am now trying to write functions that can print characters and strings on screen. By the time I start executing C code, I am in 32-bit protected mode, so I am trying to correctly calculate memory offsets from video memory address 0xb8000.
My problem starts when I try to access specific regions of video memory using a calculated offset. Since the text area is 25 rows by 80 columns, I use the formula ((row * 80) + column) * 2 since I have to have a character byte and an attribute byte. When I set row = 0 and column = 0, the X I'm trying to print is absent. Setting row = 0 and column = 1, an X appears at the upper left hand corner.
Starting with char* video_memory = 0xb8000 and repeatedly issuing video_memory++ allows me to correctly visit each byte and print a space on a black background.
This is my main code:
#include "../drivers/screen.h"
void main() {
//clear_screen();
//print_character('X', 0, 0, 0);
// Helper variables.
int row;
int column;
// We need to point at 0xB8000, where video memory resides.
unsigned char* video_memory = (unsigned char*)0xB8000;
for(row = 0; row < 25; row++) {
for(column = 0; column < 80; column++) {
// Clear the screen by printing a space on a black background.
*video_memory = ' ';
video_memory += 1;
*video_memory = 0x0F;
video_memory += 1;
}
}
// Test the offset calculation by printing at row 0, column 0 (the upper
// left corner of the screen).
row = 0;
column = 0;
// For an 80 by 25 grid. Multiply by 2 to account for the need of two bytes
// to display a character with given attributes.
int offset = ((row * 80) + column) * 2;
// Reset memory location after the loop.
video_memory = (unsigned char*)0xB8000;
// Add the offset to get the desired cell.
// THIS IS WHERE THE PROBLEM IS! Setting column = 1 prints in the first cell
// of video memory instead of the second.
video_memory += offset;
// Set character and its attributes.
*video_memory = 'X';
video_memory++;
*video_memory = 0x0F;
}
This is the console displayed when row = 0 and column = 0: The console when row and column are 0. No X appears.
This is the console when row = 0 and column = 1: The console when row is 0 and column is 1. An X appears.
This is an objdump of my kernel.c file above:
kernel.o: file format elf64-x86-64
Disassembly of section .text:
0000000000000000 <main>:
#include "../drivers/screen.h"
void main() {
0: 55 push rbp
1: 48 89 e5 mov rbp,rsp
// Helper variables.
int row;
int column;
// We need to point at 0xB8000, where video memory resides.
unsigned char* video_memory = (unsigned char*)0xB8000;
4: 48 c7 45 f8 00 80 0b mov QWORD PTR [rbp-0x8],0xb8000
b: 00
for(row = 0; row < 25; row++) {
c: c7 45 ec 00 00 00 00 mov DWORD PTR [rbp-0x14],0x0
13: eb 2f jmp 44 <main+0x44>
for(column = 0; column < 80; column++) {
15: c7 45 f0 00 00 00 00 mov DWORD PTR [rbp-0x10],0x0
1c: eb 1c jmp 3a <main+0x3a>
// Clear the screen by printing a space on a black background.
*video_memory = ' ';
1e: 48 8b 45 f8 mov rax,QWORD PTR [rbp-0x8]
22: c6 00 20 mov BYTE PTR [rax],0x20
video_memory += 1;
25: 48 83 45 f8 01 add QWORD PTR [rbp-0x8],0x1
*video_memory = 0x0F;
2a: 48 8b 45 f8 mov rax,QWORD PTR [rbp-0x8]
2e: c6 00 0f mov BYTE PTR [rax],0xf
video_memory += 1;
31: 48 83 45 f8 01 add QWORD PTR [rbp-0x8],0x1
int column;
// We need to point at 0xB8000, where video memory resides.
unsigned char* video_memory = (unsigned char*)0xB8000;
for(row = 0; row < 25; row++) {
for(column = 0; column < 80; column++) {
36: 83 45 f0 01 add DWORD PTR [rbp-0x10],0x1
3a: 83 7d f0 4f cmp DWORD PTR [rbp-0x10],0x4f
3e: 7e de jle 1e <main+0x1e>
int row;
int column;
// We need to point at 0xB8000, where video memory resides.
unsigned char* video_memory = (unsigned char*)0xB8000;
for(row = 0; row < 25; row++) {
40: 83 45 ec 01 add DWORD PTR [rbp-0x14],0x1
44: 83 7d ec 18 cmp DWORD PTR [rbp-0x14],0x18
48: 7e cb jle 15 <main+0x15>
}
}
// Test the offset calculation by printing at row 0, column 0 (the upper
// left corner of the screen).
row = 0;
4a: c7 45 ec 00 00 00 00 mov DWORD PTR [rbp-0x14],0x0
column = 0;
51: c7 45 f0 00 00 00 00 mov DWORD PTR [rbp-0x10],0x0
// For an 80 by 25 grid. Multiply by 2 to account for the need of two bytes
// to display a character with given attributes.
int offset = ((row * 80) + column) * 2;
58: 8b 55 ec mov edx,DWORD PTR [rbp-0x14]
5b: 89 d0 mov eax,edx
5d: c1 e0 02 shl eax,0x2
60: 01 d0 add eax,edx
62: c1 e0 04 shl eax,0x4
65: 89 c2 mov edx,eax
67: 8b 45 f0 mov eax,DWORD PTR [rbp-0x10]
6a: 01 d0 add eax,edx
6c: 01 c0 add eax,eax
6e: 89 45 f4 mov DWORD PTR [rbp-0xc],eax
// Reset memory location after the loop.
video_memory = (unsigned char*)0xB8000;
71: 48 c7 45 f8 00 80 0b mov QWORD PTR [rbp-0x8],0xb8000
78: 00
// Add the offset to get the desired cell.
// THIS IS WHERE THE PROBLEM IS! Setting column = 1 prints in the first cell
// of video memory instead of the second.
video_memory += offset;
79: 8b 45 f4 mov eax,DWORD PTR [rbp-0xc]
7c: 48 98 cdqe
7e: 48 01 45 f8 add QWORD PTR [rbp-0x8],rax
// Set character and its attributes.
*video_memory = 'X';
82: 48 8b 45 f8 mov rax,QWORD PTR [rbp-0x8]
86: c6 00 58 mov BYTE PTR [rax],0x58
video_memory++;
89: 48 83 45 f8 01 add QWORD PTR [rbp-0x8],0x1
*video_memory = 0x0F;
8e: 48 8b 45 f8 mov rax,QWORD PTR [rbp-0x8]
92: c6 00 0f mov BYTE PTR [rax],0xf
}
95: 90 nop
96: 5d pop rbp
97: c3 ret
I have traced over and hand checked the actual assembly instructions for my offset calculation, and they appear to be correct. I suspect the issue arises when I try to add my offset (type int) to my video memory address (type unsigned char*), but again I'm not entirely sure.
Also, I tried hard coding specific numbers for offsets. Using video_memory += 0 instead of video_memory += offset works as desired.
After more searching, I found an article on the ARM Information Center describing the use of C pointers to access specific addresses for memory mapped I/O devices.
Declaring my video_memory pointer variable as 'volatile' ensures that "the compiler always carries out the memory-accesses, rather than optimizing them out...". Apparently, according to this answer on Quora, compilers can generate instructions that overwrite data in a write buffer before that data is flushed to memory, which is where my problem occurs.
So declaring my variable as
volatile unsigned char* video_memory = 0xB8000;produces the expected results.