5. C FunctionsENGI 3655 Lab Sessions

A UNIX saleslady, Lenore,Enjoys work, but she likes the beach more.She found a good wayTo combine work and play:She sells C shells by the seashore.

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Textbook Readings Operating-System Design and

Implementation◦ Section 2.6

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Assembly Language Processor instructions labelled with

mnemonic abbreviations Advantages

◦ Very efficient code◦ Gives us very low-level control of the computer

Disadvantages◦ Writing large complex programs is very slow◦ Code not portable; the 80x86 assembly

instruction set is only valid on 80x86 CPUs For decades it was believed that “something

as complex as an operating system [has] to be written exclusively in assembly language"

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C Language High-level language

◦ Portable, more efficient for program-writing◦ But minimalist: no runtime support, efficient

mapping from instructions to machine code, and allows low-level memory access

Originally developed in 1972 at AT&T Bell Labs as a “portable Assembly” language◦ So people began rewriting their old Assembly

programs in C

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Unix History 1964: MIT, GE and Bell Labs team up to

build an OS called Multics◦ The last Multics system running was at the

Canadian Department of National Defence in Halifax, and shut down in 2000

1969: Bell Labs pulls out and start working on Unics

1970: Unix is up and running 1972: C language 1973: Unix is re-written in C language

◦ The first OS not written in Assembly◦ Since then, writing OS in C has become the norm

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Merging C and Assembly We’ll start simply by moving our infinite

loop here int KernelMain ()

{for (;;);

return 0;}

Save as “main.c”

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Merging C and Assembly We’ll need to change several things in

kernel.asm First, we need to call the main function from

the kernel, after displaying the welcome message

extern _KernelMaincall _KernelMain

◦ Replaces our infinite loop

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Merging C and Assembly Before the jmp KernelCode, add the label start, which our linker will use as a starting point

global startstart:jmp KernelCode

Remove the org 0x100000 line, the linker will take care of that

And that’s it!◦ Our Assembly kernel now calls a C kernel◦ We can write our OS in C!

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Display Functions Last week we created four Mode-7 display

functions in Assembly for our kernel◦ ClearScreen◦ DisplayString◦ DisplayChar◦ MoveCursor

We’ll rewrite them in C◦ Along with a few extra functions we’ll need

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Writing an OS in C We are writing everything from scratch

◦ We can’t use pre-made standard libraries◦ Any definition, function, or included header file we

use, we have to make ourselves Data type hiding is key

◦ A major problem with our Assembly kernel is that it was dependent on 80x86 register names and sizes

◦ We want our C kernel to be hardware-independent◦ Therefore, we cannot use variable types that can

vary from system to system and compiler to compiler

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Interface We’ll use an interface

◦ Define new names for each data type we’ll use that might cause compatibility issues

typedef int int32_t; ◦ Put them all in header files◦ Use our own names in the OS code◦ When we switch to a different system with

different data types, we’ll simply change the relevant header files

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Interface To spare us from making hundreds of type

definitions, some header files are on the website◦ You should take a look in each of them later

_null.h: Defines the NULL constant size_t.h: Defines the unsigned type stdint.h: Defines various forms of integers ctype.h: ASCII-based character macros va_list.h: Variable-length argument list macros stdarg.h: Standard argument-list-handling

macros

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Interface We’ll store these six header files away

somewhere, for neatness ◦ Somewhere our compiler will find them◦ Create an include subfolder called “include”◦ Later, we’ll specify to our C compiler to check that

folder

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Display Functions We’re creating a set of functions to display

characters and strings in Mode 7◦ “mode7.c” and “mode7.h”

Begin with some useful global constants and variables#define VID_MEMORY 0xB8000#define COLS 80#define LINES 25static uint32_t _xPos=0, _yPos=0;static size_t _color=0;

Notice the use of our interfaces

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DisplayChar Recall from Assembly: the char (without the

attributes) is just one byte, and unsigned◦ Again from stdint.h, we have:

typedef unsigned char uint8_t; The display char function needs to

◦ Skip the zero (empty) char◦ Change line at the special “change line” char◦ Change line after the last column◦ Write the char at the right place in memory,

followed by its attributes

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DisplayCharvoid DisplayChar(uint8_t c){ Skip the zero (empty) char Change line at the special “change line” char Change line at the end of the column Write the char at the right place in memory, followed

by its attributes}

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DisplayCharvoid DisplayChar(uint8_t c){ Skip the zero (empty) charif (c == 0)return;

Change line at the special “change line” char Change line at the end of the column Write the char at the right place in memory,

followed by its attributes}

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DisplayCharvoid DisplayChar(uint8_t c){ Skip the zero (empty) char Change line at the special “change line” charif (c == '\n') {_yPos++;_xPos=0;if (_yPos >= LINES)ScrollUp();MoveCursor();return; }

Change line at the end of the column Write the char at the right place in memory,

followed by its attributes}

You can add other special characters this way, like \t and \b.

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DisplayCharvoid DisplayChar(uint8_t c){ Skip the zero (empty) char Change line at the special “change line” char Change line at the end of the columnif (_xPos >= COLS) {_yPos++;_xPos=0;if (_yPos >= LINES)ScrollUp();}

Write the char at the right place in memory, followed by its attributes

}

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DisplayCharvoid DisplayChar(uint8_t c){ Skip the zero (empty) char Change line at the special “change line” char Change line at the end of the column Write the char at the right place in memory, followed

by its attributesuint8_t* p = (uint8_t*)VID_MEMORY + _yPos * COLS * 2 + (_xPos++)*2;*p++ = c;*p =_color;MoveCursor();

}

Notice that, in addition to changing lines at the end of COLS, this function scrolls up at the end of LINES

We’ll need to write that functionvoid ScrollUp(){

◦ Loop through video memory, over the area corresponding to lines 0-23, copying the data from lines 1-24 over it

◦ Clear line 24◦ Update current position to last line

}

ScrollUp

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DisplayString Like for the Assembly equivalent, this function will

break up a string into characters and call DisplayChar for each one

void DisplayString(uint8_t *str){if (!str)

return;

size_t i;for (i=0; i < strlen(str); i++)

DisplayChar(str[i]);}

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DisplayString One difference is that the Assembly function

displayed chars in an endless loop until it hit the 0 char

In C, we evaluate the length of the string ◦ But we need to write our own strlen function

size_t strlen ( const uint8_t* str ) {size_t len=0;while (str[len++]);return len;

}

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ClearScreen Unlike Assembly, this function allows users

to specify which colour to clear the screen to◦ Aside from that, the function works like in

Assembly

void ClearScreen (const size_t c) {

You have to write that function}

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MoveCursor That one is a problem to write in C Recall: We had to access the CRTC registers

through the controller’s ports◦ Only a handful of commands in Assembly

mov al, 0x0f mov dx, 0x03D4 out dx, al

◦ High-level languages like C don’t give us this kind of control

Solution: call Assembly functions through C

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MoveCursor In Assembly, we had four “out” commands

with different arguments in the registers So we’ll have one C function to call the

“out” command with given arguments In port.c and port.hvoid outport(uint16_t port, uint8_t data){ __asm__ __volatile__ ("out %1, %0" : : "dN" (port), "a" (data));

}

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MoveCursor Then a function to call that one four times with the

right argumentsvoid MoveCursor(){ uint32_t cPos; cPos = _yPos * COLS + _xPos;

outport(0x3D4, 14); outport(0x3D5, cPos >> 8); outport(0x3D4, 15); outport(0x3D5, cPos);}

After we’ve rewritten all the old Assembly functions, it’s time for our first new function

This function will display a positive base-10 integer◦ That will be very useful as we build our OS

void DisplayInteger(uint32_t n) But how, since our DisplayChar can only

display individual characters (uint8_t)◦ We will need to convert the number digit by digit

DisplayInteger

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void DisplayInteger(uint32_t n){ Handle the simple n=0 case Split the number right-to-left in individual

digits and store the ASCII value Display each digit in left-to-right order}

DisplayInteger

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void DisplayInteger(uint32_t n){ Handle the simple n=0 caseif (n == 0)

{ DisplayChar('0'); return; } Split the number right-to-left in individual digits and

store the ASCII value Display each digit in left-to-right order}

DisplayInteger

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void DisplayInteger(uint32_t n){ Handle the simple n=0 case Split the number right-to-left in individual digits and store the ASCII valueuint32_t acc = n;uint8_t c[32];

uint32_t i = 0; while (acc > 0) { c[i] = '0' + acc%10; acc /= 10; i++; } i--; Display each digit in left-to-right order}

DisplayInteger

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void DisplayInteger(uint32_t n){ Handle the simple n=0 case Split the number right-to-left in individual digits and store the

ASCII value Display each digit in left-to-right order while (1) {DisplayChar(c[i]);if (i == 0) break;i--;

}}

DisplayInteger

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Writing an OS in C Now we have our four Assembly display

functions in C, and more Don’t forget

◦ Write the function definitions in “mode7.h”extern void DisplayChar(uint8_t c);◦ Include the right header files◦ Include “mode7.h” in “main.c” so that we can

display stuff!

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Lab Assignment Add a few more functions to get more display

options◦ ScrollUp

As explained in the slides, a function that copies lines 1-24 onto lines 0-23, then clears line 24 and update the position

◦ ClearScreen As explained in the slides, a function that clears the screen

to a background function passed in argument◦ SetColor

A function to change the character parameter variable◦ GotoXY

A function that moves the display & cursor to new (x,y) coordinates, so that the next character we write is at that position

And some interface functions that will be useful to display stuff◦ GetColor

Return the current value of the character parameter variable◦ GetWidth

Return the number of columns in the current display mode◦ GetHeight

Return the number of rows in the current display mode◦ GetXPos

Returns the cursor’s current X position◦ GetYPos

Returns the cursor’s current Y position

Lab Assignment

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Lab Assignment Don’t forget to include the functions in this

presentation◦ DisplayChar◦ DisplayString◦ strlen◦ MoveCursor◦ DisplayInteger

Then update your main function to display a friendly, colourful welcome message for your OS somewhere on the screen

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Kernel#include <mode7.h>int KernelMain () {//Clear the screen//Your welcome message here

SetColor(0x12);GotoXY(78,24);DisplayString("+-+-+-+-"); for (;;);return 0;}

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Compiling and Linking What we’ll need An Assembly compiler

◦ We already have NASM A C compiler

◦ DJGPP: DOS port of GCC, the GNU Compiler A linker DOSBOX

◦ 32-bit DOS environment simulation, for GCC to work

Available on the course website

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Compiling We will compile the Assembly and C files separately

into objects We use a different NASM option to compile the kernel

into an objectnasm -f aout -o kernel.o kernel.asm

GCC and LD are old 32-bit C compilers and linkers, won’t work in 64-bits Windows

We’ll run them in DOSBOX

Z:\>MOUNT C C:\Users\Richy\Desktop\OSLabZ:\>MOUNT D D:\Z:\> C:

Note: Dosbox uses 8.3 file name format, and does not allow spaces

Compiling

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Compiling We compile the C file using DJGPP

set DJGPP=djgpp.env

gcc -Wall -O -fstrength-reduce -fomit-frame-pointer -finline-functions -nostdinc -fno-builtin -I./include -c -o main.o main.c◦ Compile only (no linking), without standard C libraries,

and include files are in the “include” subfolder Compile mode7.c and port.c separately, using the

same command as main.c

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Linking We combine the object files into our kernel

using the linkerld -T link.ld -o KERNEL.BIN kernel.o main.o mode7.o port.o

Link using the details specified in link.ld◦ If you look at the file, you will find

ENTRY(start)◦ The start label I made you add

phys = 0x00100000;◦ The replacement for the ORG command we deleted

OUTPUT_FORMAT("binary")◦ Command to create a plain binary file

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Testing Now we have the kernel.bin file that

combines both Assembly and C code We can copy it to our USB stick and test

We do have a lot of commands to type each time…◦ And we’ll add a lot more before the end of the

term◦ Tip: Make a .bat file with all the commands

written out, and just run that