Ch 01: Introduction to Assembly Language

; ZF and SF Example
mov ax, 5
sub ax, 5     ; AX = 0 -> ZF = 1, SF = 0
mov ax, 3
sub ax, 5     ; AX = -2 (0xFFFE) -> ZF = 0, SF = 1

; Enabling and Disabling Hardware Interrupts in Assembly
cli    ; Clear Interrupt Flag (IF = 0, disable interrupts)
; ... critical section ...
sti    ; Set Interrupt Flag (IF = 1, enable interrupts)

; Signed Arithmetic Overflow Example
mov al, +127   ; Max positive 8-bit signed integer (0x7F)
add al, 1      ; AL becomes 128 (0x80 = -128 in 2's complement); OF = 1

; Theoretical IP Redirection
; Normal CS:IP points to valid instruction stream
; Hijacked CS:IP points to malicious code segment offset

; Data Movement Examples in 8086 Assembly
mov ax, bx    ; Copy contents of BX into AX
mov ax, 19    ; Load immediate value 19 into AX
mov ax, 89    ; Load immediate value 89 into AX (equivalent to LDA 89)

; 8086 Data Registers Usage Example
add ax, ax      ; AX = AX + AX
mov bl, [0x0500]; Load byte from memory offset into BL
mov cx, 0x0005  ; Set loop counter
mul bx          ; DX:AX = AX * BX

; Example string destination indexing using ES:DI
mov ax, 0xB800
mov es, ax      ; Point ES to video display memory
xor di, di      ; DI = 0 (top-left screen position ES:DI)
mov ax, 0x0720  ; Blank space character with normal attribute
stosw           ; Store word into [ES:DI]