Assembly Language Notes

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MASM Code

.386

DATA    SEGMENT USE16
    ...
DATA    ENDS

STACK   SEGMENT UES16 STACK
    DB xxx DUP(0)
STACK   ENDS

CODE    SEGMENT USE16
ASSUME  CS:CODE, DS:DATA, SS:STACK

START:
    MOV     AX, DATA
    MOV     DS, AX
    ...
    MOV     AH, 4CH
    INT     21H

CODE    ENDS
    END START


IOSTR   MACRO A, N
    ...
    ENDM

DOS System Calls

  • Keyboard input 
      MOV     AH, 1
  INT     21H
  ; send ASCII to AL
  • Character output 
      MOV     DL, CHAR    ; send CHAR to output to DL
  MOV     AH, 2
  INT     21H
  • String output 
      LEA     DX, STR     ; pointer to a '$' terminated string
  MOV     AH, 9
  INT     21H
  • String input 
      LEA     DX, BUF     ; BUF with structure (count)(input_length)(buffer)
  MOV     AH, 10
  INT     21H

Physical Address

(SS) = 1000H,
[EBP + ESI] = SS << 4 + EBP + ESI (real mode)

段首址左移 4 位后再加。

Comparison

  Signed Unsigned
Equal to JE JE
Not Equal to JNE JNE
Greater than (or Equal to) JG(E) -
Less than (or Equal to) JL(E) -
Above (or Even) - JA(E)
Below (or Even) - JB(E)

Addressing and Registers

  • Register Indirect Addressing - 寄存器间接寻址
    • Format:
      • [R]
    • Register Availability:
      • All 32-bit registers
      • BX, DI, SI, BP
    • Default Segment:
      • BP, EBP, ESP goes to stack segment
  • Indexed Addressing - 变址寻址
    • Format:
      • [R*F+V], [R*F]+V, V[R*F]
    • Register Availability:
      • All 32-bit registers
    • Limitations:
      • F must be 1 when R is 16-bit register or ESP
    • Default Bit Width:
      • Depend on V if V is variable or label
  • Based Indexed Addressing - 基址加变址寻址
    • NOTE: With two registers.
    • Format:
      • [BR+IR*F+V], V[BR+IR*F], V[BR][IR*F]
    • Register Availability:
      • All 32-bit registers (IR can’t be ESP)
      • BR: BX, BP
      • IR: SI, DI
    • Default Segment:
      • When BR is BX, goes to data segment
      • When BR is BP, ESP, EBP, goes to stack segment

Bit Length

  • PUSH AL 非法：压/出栈操作数应为 16/32 位。

Sub-Process

  • INVOKE STDCALL：从右到左顺序进栈。

INVOKE ITOA, 1234H, 10, OFFSET BUF

RADIX_S PROC NEAR STDCALL USES EBX EDX SI, LpResult, Radix:DWORD, NUM:DWORD
    LOCAL   COUNT:WORD
    ...
RADIX_S     ENDP

stack structure:

Note: Stack grows from TOP, goes DOWN.

address content comment
high …  
  00H  
+8 1FH OFFSET BUF
  00H  
+6 0AH 10 as WORD
  12H  
+4 34H 1234H
  00H  
+2 10H IP to next line under INVOKE
  (BP-H)  
0 (BP-L) (BP)=(SP)
low …  
content
low
(SI)
 
(EDX)
 
(EBX)
COUNT
(BP)
(IP)
LpResult
 
Radix
 
NUM
high

CALL only pushes IP (for FAR, push CS then IP), finally pushes BP.

STRCMP  PROC NEAR
    PUSH    BP
    MOV     BP, SP
    MOV     SI, [BP+4]      ; take first parameter to SI
                            ; +4 = sizeof(BP) + sizeof(IP)
    ...

Interupt Service Process

Number 10H: 4 bits, from 0:40H to 0:43H (40H = 10H * (4)_10)

  • Interupt is done by CPU.

What happened when an interupt process is invoked?

Under real mode, when executing INT m

  1. PUSHF - save flag register status
  2. push CS and IP successively
  3. 4 * m to IP, 4 * m + 2 to CS

Sub-Process

  • NEAR: CS is not pushed with NEAR jumps.
  • STDCALL: parameters are pushed into stack from right to left.

Win32 Window Programming

Tasks of WinMain()

  1. Initialize window parameters, register the window class
  2. Create window instance
  3. Load resources, e.g. menus
  4. Message looooooop
  5. On recieving quit message, quit

Clever Hacks

  • n << 1 == n * 2 == n + n