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"

巴特西

LinuxAsm#Chapter10

Dividing and Conquering

Book: Assembly Language step by step

Complexity kills programs.

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

Calling and Returning

Saving the Caller's Registers

Table Tricks

Local Labels and the Lengths of Jumps

Type of Jumps

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:

lib.asm:

Simple Cursor Control in the Linux Console

Creating and Using Macros

Local Labels Within Macros

Macro Libraries As Include Files

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