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Add file handling lessons 22-28.
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‎code/lesson22/create.asm‎

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; Create
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; Compile with: nasm -f elf create.asm
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; Link with (64 bit systems require elf_i386 option): ld -m elf_i386 create.o -o create
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; Run with: ./create
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%include 'functions.asm'
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SECTION .data
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filename db 'readme.txt', ; the filename to create
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SECTION .text
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global _start
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_start:
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mov ecx, 0777 ; set all permissions to read, write, execute
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mov ebx, filename ; filename we will create
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mov eax, 8 ; invoke SYS_CREAT (kernel opcode 8)
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int 80h ; call the kernel
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call quit ; call our quit function

‎code/lesson22/functions.asm‎

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;------------------------------------------
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; int atoi(Integer number)
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; Ascii to integer function (atoi)
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atoi:
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push ebx ; preserve ebx on the stack to be restored after function runs
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push ecx ; preserve ecx on the stack to be restored after function runs
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push edx ; preserve edx on the stack to be restored after function runs
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push esi ; preserve esi on the stack to be restored after function runs
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mov esi, eax ; move pointer in eax into esi (our number to convert)
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mov eax, 0 ; initialise eax with decimal value 0
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mov ecx, 0 ; initialise ecx with decimal value 0
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.multiplyLoop:
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xor ebx, ebx ; resets both lower and uppper bytes of ebx to be 0
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mov bl, [esi+ecx] ; move a single byte into ebx register's lower half
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cmp bl, 48 ; compare ebx register's lower half value against ascii value 48 (char value 0)
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jl .finished ; jump if less than to label finished
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cmp bl, 57 ; compare ebx register's lower half value against ascii value 57 (char value 9)
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jg .finished ; jump if greater than to label finished
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cmp bl, 10 ; compare ebx register's lower half value against ascii value 10 (linefeed character)
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je .finished ; jump if equal to label finished
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cmp bl, 0 ; compare ebx register's lower half value against decimal value 0 (end of string)
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jz .finished ; jump if zero to label finished
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sub bl, 48 ; convert ebx register's lower half to decimal representation of ascii value
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add eax, ebx ; add ebx to our interger value in eax
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mov ebx, 10 ; move decimal value 10 into ebx
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mul ebx ; multiply eax by ebx to get place value
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inc ecx ; increment ecx (our counter register)
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jmp .multiplyLoop ; continue multiply loop
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.finished:
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mov ebx, 10 ; move decimal value 10 into ebx
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div ebx ; divide eax by value in ebx (in this case 10)
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pop esi ; restore esi from the value we pushed onto the stack at the start
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pop edx ; restore edx from the value we pushed onto the stack at the start
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pop ecx ; restore ecx from the value we pushed onto the stack at the start
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pop ebx ; restore ebx from the value we pushed onto the stack at the start
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ret
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;------------------------------------------
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; void iprint(Integer number)
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; Integer printing function (itoa)
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iprint:
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push eax ; preserve eax on the stack to be restored after function runs
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push ecx ; preserve ecx on the stack to be restored after function runs
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push edx ; preserve edx on the stack to be restored after function runs
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push esi ; preserve esi on the stack to be restored after function runs
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mov ecx, 0 ; counter of how many bytes we need to print in the end
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.divideLoop:
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inc ecx ; count each byte to print - number of characters
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mov edx, 0 ; empty edx
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mov esi, 10 ; mov 10 into esi
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idiv esi ; divide eax by esi
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add edx, 48 ; convert edx to it's ascii representation - edx holds the remainder after a divide instruction
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push edx ; push edx (string representation of an intger) onto the stack
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cmp eax, 0 ; can the integer be divided anymore?
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jnz .divideLoop ; jump if not zero to the label divideLoop
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.printLoop:
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dec ecx ; count down each byte that we put on the stack
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mov eax, esp ; mov the stack pointer into eax for printing
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call sprint ; call our string print function
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pop eax ; remove last character from the stack to move esp forward
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cmp ecx, 0 ; have we printed all bytes we pushed onto the stack?
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jnz .printLoop ; jump is not zero to the label printLoop
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pop esi ; restore esi from the value we pushed onto the stack at the start
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pop edx ; restore edx from the value we pushed onto the stack at the start
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pop ecx ; restore ecx from the value we pushed onto the stack at the start
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pop eax ; restore eax from the value we pushed onto the stack at the start
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ret
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;------------------------------------------
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; void iprintLF(Integer number)
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; Integer printing function with linefeed (itoa)
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iprintLF:
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call iprint ; call our integer printing function
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push eax ; push eax onto the stack to preserve it while we use the eax register in this function
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mov eax, 0Ah ; move 0Ah into eax - 0Ah is the ascii character for a linefeed
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push eax ; push the linefeed onto the stack so we can get the address
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mov eax, esp ; move the address of the current stack pointer into eax for sprint
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call sprint ; call our sprint function
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pop eax ; remove our linefeed character from the stack
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pop eax ; restore the original value of eax before our function was called
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ret
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;------------------------------------------
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; int slen(String message)
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; String length calculation function
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slen:
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push ebx
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mov ebx, eax
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.nextchar:
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cmp byte [eax], 0
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jz .finished
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inc eax
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jmp .nextchar
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.finished:
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sub eax, ebx
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pop ebx
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ret
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;------------------------------------------
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; void sprint(String message)
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; String printing function
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sprint:
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push edx
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push ecx
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push ebx
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push eax
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call slen
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mov edx, eax
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pop eax
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mov ecx, eax
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mov ebx, 1
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mov eax, 4
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int 80h
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pop ebx
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pop ecx
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pop edx
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ret
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;------------------------------------------
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; void sprintLF(String message)
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; String printing with line feed function
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sprintLF:
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call sprint
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push eax
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mov eax, 0AH
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push eax
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mov eax, esp
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call sprint
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pop eax
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pop eax
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ret
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;------------------------------------------
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; void exit()
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; Exit program and restore resources
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quit:
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mov ebx, 0
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mov eax, 1
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int 80h
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ret
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‎code/lesson22/makefile‎

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FILENAME=create
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all: cleanup
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nasm -f elf $(FILENAME).asm
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ld -m elf_i386 $(FILENAME).o -o $(FILENAME)
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./$(FILENAME)
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cleanup:
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rm -f $(FILENAME).o
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rm -f $(FILENAME)
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rm -f 'readme.txt'

‎code/lesson23/functions.asm‎

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;------------------------------------------
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; int atoi(Integer number)
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; Ascii to integer function (atoi)
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atoi:
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push ebx ; preserve ebx on the stack to be restored after function runs
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push ecx ; preserve ecx on the stack to be restored after function runs
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push edx ; preserve edx on the stack to be restored after function runs
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push esi ; preserve esi on the stack to be restored after function runs
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mov esi, eax ; move pointer in eax into esi (our number to convert)
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mov eax, 0 ; initialise eax with decimal value 0
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mov ecx, 0 ; initialise ecx with decimal value 0
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.multiplyLoop:
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xor ebx, ebx ; resets both lower and uppper bytes of ebx to be 0
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mov bl, [esi+ecx] ; move a single byte into ebx register's lower half
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cmp bl, 48 ; compare ebx register's lower half value against ascii value 48 (char value 0)
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jl .finished ; jump if less than to label finished
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cmp bl, 57 ; compare ebx register's lower half value against ascii value 57 (char value 9)
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jg .finished ; jump if greater than to label finished
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cmp bl, 10 ; compare ebx register's lower half value against ascii value 10 (linefeed character)
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je .finished ; jump if equal to label finished
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cmp bl, 0 ; compare ebx register's lower half value against decimal value 0 (end of string)
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jz .finished ; jump if zero to label finished
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sub bl, 48 ; convert ebx register's lower half to decimal representation of ascii value
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add eax, ebx ; add ebx to our interger value in eax
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mov ebx, 10 ; move decimal value 10 into ebx
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mul ebx ; multiply eax by ebx to get place value
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inc ecx ; increment ecx (our counter register)
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jmp .multiplyLoop ; continue multiply loop
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.finished:
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mov ebx, 10 ; move decimal value 10 into ebx
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div ebx ; divide eax by value in ebx (in this case 10)
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pop esi ; restore esi from the value we pushed onto the stack at the start
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pop edx ; restore edx from the value we pushed onto the stack at the start
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pop ecx ; restore ecx from the value we pushed onto the stack at the start
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pop ebx ; restore ebx from the value we pushed onto the stack at the start
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ret
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;------------------------------------------
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; void iprint(Integer number)
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; Integer printing function (itoa)
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iprint:
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push eax ; preserve eax on the stack to be restored after function runs
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push ecx ; preserve ecx on the stack to be restored after function runs
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push edx ; preserve edx on the stack to be restored after function runs
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push esi ; preserve esi on the stack to be restored after function runs
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mov ecx, 0 ; counter of how many bytes we need to print in the end
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.divideLoop:
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inc ecx ; count each byte to print - number of characters
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mov edx, 0 ; empty edx
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mov esi, 10 ; mov 10 into esi
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idiv esi ; divide eax by esi
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add edx, 48 ; convert edx to it's ascii representation - edx holds the remainder after a divide instruction
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push edx ; push edx (string representation of an intger) onto the stack
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cmp eax, 0 ; can the integer be divided anymore?
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jnz .divideLoop ; jump if not zero to the label divideLoop
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.printLoop:
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dec ecx ; count down each byte that we put on the stack
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mov eax, esp ; mov the stack pointer into eax for printing
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call sprint ; call our string print function
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pop eax ; remove last character from the stack to move esp forward
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cmp ecx, 0 ; have we printed all bytes we pushed onto the stack?
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jnz .printLoop ; jump is not zero to the label printLoop
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pop esi ; restore esi from the value we pushed onto the stack at the start
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pop edx ; restore edx from the value we pushed onto the stack at the start
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pop ecx ; restore ecx from the value we pushed onto the stack at the start
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pop eax ; restore eax from the value we pushed onto the stack at the start
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ret
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;------------------------------------------
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; void iprintLF(Integer number)
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; Integer printing function with linefeed (itoa)
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iprintLF:
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call iprint ; call our integer printing function
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push eax ; push eax onto the stack to preserve it while we use the eax register in this function
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mov eax, 0Ah ; move 0Ah into eax - 0Ah is the ascii character for a linefeed
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push eax ; push the linefeed onto the stack so we can get the address
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mov eax, esp ; move the address of the current stack pointer into eax for sprint
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call sprint ; call our sprint function
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pop eax ; remove our linefeed character from the stack
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pop eax ; restore the original value of eax before our function was called
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ret
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;------------------------------------------
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; int slen(String message)
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; String length calculation function
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slen:
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push ebx
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mov ebx, eax
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.nextchar:
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cmp byte [eax], 0
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jz .finished
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inc eax
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jmp .nextchar
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.finished:
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sub eax, ebx
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pop ebx
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ret
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;------------------------------------------
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; void sprint(String message)
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; String printing function
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sprint:
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push edx
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push ecx
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push ebx
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push eax
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call slen
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mov edx, eax
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pop eax
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mov ecx, eax
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mov ebx, 1
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mov eax, 4
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int 80h
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pop ebx
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pop ecx
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pop edx
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ret
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;------------------------------------------
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; void sprintLF(String message)
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; String printing with line feed function
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sprintLF:
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call sprint
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push eax
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mov eax, 0AH
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push eax
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mov eax, esp
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call sprint
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pop eax
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pop eax
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ret
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;------------------------------------------
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; void exit()
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; Exit program and restore resources
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quit:
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mov ebx, 0
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mov eax, 1
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int 80h
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ret
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‎code/lesson23/makefile‎

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FILENAME=write
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all: cleanup
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nasm -f elf $(FILENAME).asm
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ld -m elf_i386 $(FILENAME).o -o $(FILENAME)
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./$(FILENAME)
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cleanup:
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rm -f $(FILENAME).o
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rm -f $(FILENAME)
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rm -f 'readme.txt'

‎code/lesson23/write.asm‎

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; Write
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; Compile with: nasm -f elf write.asm
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; Link with (64 bit systems require elf_i386 option): ld -m elf_i386 write.o -o write
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; Run with: ./write
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%include 'functions.asm'
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SECTION .data
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filename db 'readme.txt', 0h ; the filename to create
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contents db 'Hello world!', 0h ; the contents to write
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SECTION .text
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global _start
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_start:
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mov ecx, 0777 ; code continues from lesson 22
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mov ebx, filename
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mov eax, 8
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int 80h
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mov edx, 12 ; number of bytes to write - one for each letter of our contents string
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mov ecx, contents ; move the memory address of our contents string into ecx
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mov ebx, eax ; move the file descriptor of the file we created into ebx
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mov eax, 4 ; invoke SYS_WRITE (kernel opcode 4)
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int 80h ; call the kernel
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call quit ; call our quit function

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