Lecture 19–24: Procedures and Stack Management
➤ What is a Procedure?
• A procedure (like a function) is a named block of code that performs a specific task.
• Use CALL to jump to a procedure and RET to return.
• Saves the return address on the stack when called.
➤ Passing Parameters
• Parameters can be passed:
o Directly using registers
o Indirectly via memory
o By stack (most flexible)
• When using stack:
o PUSH parameters
o Inside procedure, access via BP (Base Pointer)
➤ Stack Usage
• Stack uses LIFO (Last In First Out)
• Instructions:
o PUSH: Adds data to stack
o POP: Removes data from stack
• BP is used to access parameters and local variables:
o [BP+4] = first pushed parameter (after return address)
➤ Local Variables
• Space is created on stack using SUB SP, n
• Accessed using [BP - offset]
• Must be cleaned using ADD SP, n before RET
➤ Saving/Restoring Registers
• Save important registers before procedure runs:
o PUSH AX … Do work … POP AX
➤ Nested Procedures
• A procedure can call another
• Each CALL pushes return address
• Proper stack management is crucial to avoid errors
Lecture 25–30: Macros and Conditional Assembly
➤ What is a Macro?
• A macro is a textual substitution, not an actual procedure call.
• No CALL/RET overhead; code is inserted inline.
• Defined using:
asm
CopyEdit
MyMacro MACRO
; code here
ENDM
➤ Macro vs Procedure
Feature Macro Procedure
Speed Faster Slower
Size Larger Smaller
Uses Stack? No Yes
Reusable? Less More
➤ Conditional Assembly
• Control which code gets assembled using:
o IF, ELSE, ENDIF
• Example:
asm
CopyEdit
IF CONST EQ 1
; Code for option 1
ELSE
; Code for option 2
ENDIF
➤ Looping in Macros
• Use:
o REPT n: repeat block n times
o IRP: loop over parameters
o IRPC: loop over characters in a string
Lecture 31–35: String Instructions
➤ String Instructions Basics
• Used to operate on arrays or strings.
• Registers:
o SI: Source Index
o DI: Destination Index
o CX: Counter
o DS, ES: Segment pointers
➤ Important Instructions
• MOVSB / MOVSW: Move byte/word
• LODSB / LODSW: Load from SI to AL/AX
• STOSB / STOSW: Store AL/AX to DI
• SCASB / SCASW: Compare AL/AX with DI
• CMPSB / CMPSW: Compare [SI] with [DI]
➤ REP Prefixes
• REP: Repeat instruction CX times
• REPE/REPZ: Repeat while equal / zero
• REPNE/REPNZ: Repeat while not equal / not zero
Lecture 36–39: Interrupts and BIOS/DOS Functions
➤ What is an Interrupt?
• A signal to CPU to stop current task and run another (usually I/O or system-level)
• In Assembly, we use INT instruction.
➤ Common Interrupts
• DOS Interrupt (INT 21h):
o AH=01h: Read character
o AH=09h: Display string
• BIOS Interrupts:
o INT 10h: Video
o INT 16h: Keyboard input
➤ Using INT 21h Example
asm
CopyEdit
MOV AH, 09h
MOV DX, OFFSET msg
INT 21h
Lecture 40–42: File Handling in Assembly
➤ DOS File Functions (INT 21h)
• Open File: AH=3Dh
• Read File: AH=3Fh
• Write File: AH=40h
• Close File: AH=3Eh
➤ Example File Read
asm
CopyEdit
MOV AH, 3Dh ; Open file
MOV DX, OFFSET filename
INT 21h
; File handle returned in AX
➤ File Pointer (Seek)
• AH = 42h, AL defines origin:
o 0: Beginning
o 1: Current
o 2: End
Lecture 43–45: Mixing Assembly with C/C++
➤ Inline Assembly in C
• Use asm {} or __asm to embed ASM code inside C.
• C can:
o Call ASM functions
o Pass values via registers or stack
➤ Calling ASM from C
• Define extern "C" to avoid name mangling
• Match calling convention
➤ Calling C from ASM
• Push parameters in reverse order
• Use proper function labels from C object file
• Clean the stack after call
Final Tips for Understanding
Topic Focus Area
Procedures CALL/RET, BP usage, stack
Macros Fast, inline, no stack use
String Ops REP family, SI/DI/CX
Interrupts INT 21h (DOS), INT 10h (BIOS)
File Handling INT 21h AH codes, handle management
C & ASM Parameter passing, naming conventions
