LinuxAsm#Chapter10

Dividing and Conquering

• Book: Assembly Language step by step

• Complexity kills programs.

• Remember to use comment headers.(Comment is very very important!)

More Than 25 lines and you're doing too much in one procedure.Split it up.

Calling and Returning

call LoadBuff
...
loadBuff:
push eax ; a1
push ebx ; a2
push edx ; a3
mov eax, 3 ; sys_read call
mov ebx, 0 ; File Descriptor 0: stdin
mov ecx, Buff ; offset of the buffer to read to
mov edx, BUFFLEN ; number of bytes to read at one pass
int 80h ; sys_read
mov ebp, eax
xor ecx, ecx
pop edx ; b1
pop ebx ; b2
pop eax ; b3
ret

• a1~a3 and b1~b3 are store of stack
• "xor ecx, ecx" is faster than "mov ecx, 0"

Saving the Caller's Registers

pushad
...
popad

Table Tricks

DumpLin db " 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 "
DUMPLEN equ $-DumpLin
ASCLin db "|................|", 10
ASCLEN equ $-ASCLin
FULLLEN equ $-DumpLin

lea edi, [edx*2+edx] ; trick to calculate 3*edx

Local Labels and the Lengths of Jumps

Scan: 
xor eax, eax
...
.modTest:
test esi, 0000000Fh
...

• "Scan" is a nonlocal label.(global label)
• ".modTest" is a local label.
• Local labels are local to the first nonlocal label that precedes them in the code.
• A local label cannot be referenced higher in the source code file than the global label that owns it.
• Local labels are not accessible as breakpoints from the command-line interface of GDB.
• Good habits: local labels and all jumps to them should occur within a single screen of code.

Type of Jumps

jne Scan ; Short jump, to within 127 bytes in either direction
jne near Scan ; Near jump, anywhere in the current code segment

Building External Procedure Libraries

• Each executable file can only contain one _start: label. External modules do not contain _start:

• External modules do not return to Linux.(Only the main program module can make sys_exit INT 80h call)

main-program.asm:

section .text
extern ClearLine
global _start
_start:
...
call ClearLine
...

lib.asm:

section .text
global ClearLine
...
ClearLine:
...

Rules:

• "extern" to declare all the labels that don't belong to the current file.
• "global" to declare all the labels in the current file needed by other files.

Simple Cursor Control in the Linux Console

[section .data]    
PositionTerm db 27, "[01;01H"  ;  <ESC>[<Y>;<X>H  -This sequences move the cursor to (X, Y)
ClearTerm db 27, "[2J"  ;  <ESC>[2J    -This sequences clears the display.
GreenBack db 27, "[42m"  ; <ESC>[42m  -turns the consoles background green

For more details about console escape codes: man console_codes

Creating and Using Macros

%macro WriteStr 2 ; 2 arguments
    push eax
    push ebx
    mov ecx, %1 ; %1 invokes the first argument (Prompt)
    mov edx, %2 ; %2 invokes the second argument (PROMPTLEN)
    mov eax, 4
    mov edx, 1
    int 80h
    pop ebx
    pop eax
%endmacro
....
....
WriteStr Prompt, PROMPTLEN
; When a macro is invoked, its arguments are separated by commas.

Local Labels Within Macros

%macro UpCase 2
    mov edx, %1
    mov ecx, %2
%%IsLC:
    cmp byte [edx+ecx-1], 'a'
    jb %%Bump
    cmp byte [edx+ecx-1], 'z'
    ja %%Bump
    sub byte [edx+ecx-1], 20h
%%Bump:
    dec ecx
    jnz %%IsLC
%endmacro

Macro Libraries As Include Files

%include "mylib.mac"