String Instructions

1
String Instructions

• MOVS move string
• CMPS compare string
• SCAS scan string
• STOS store string
• LODS load string

2
MOVS

• Move string data Copy a memory byte, word or
  double word from one memory location to another.
• Implied operands MOVS DI, SI

3
Example of MOVS

• ES and DS both must contain address of data
  segments, done by three instructions
• Mov ax, _at_data
• Mov ds, ax
• Mov es, ax
• Mov si, offset source addr of source in si
• Mov di, offset dest addr of dest in di
• Movsb copy byte from source to destn
• Moves one byte from source pointed by si to
  source pointed by di

4
Repeat instructions

• REP repeat while cx / 0
• REPZ, REPE repeat while the zero flag is set
  and cx gt0
• REPNZ, REPNE repeat while the zero flag is
  clear and cx gt0

5
MOVS with repeat

• Mov si, offset str1 si ? addr of str1
• Mov di, offset str2 di ? addr of str2
• Mov cx, 10 cx ? of bytes to be copied
• Rep movsb copies 10 bytes from str1 to str2

6
Direction Flag

• CLD clear direction flag
• STD sets direction flag
• If direction flag is cleared, registers SI and
  DI are both incremented, and n bytes are copied
  from left to right (low-high addr)
• If direction flag is set, registers SI and DI
  are both decremented, and n bytes are copied from
  right to left (low-high addr)

7
MOVS_ (different forms)

• MOVSB move string byte
• MOVSW move string word
• MOVSD move string double word
• Instruction auto change in si / di (inc or
  dec)
• movsb 1
• movsw 2
• movsd 4

8
CMPSb Compare string byte

• Mov si, offset source
• Mov di, offset dest
• Mov cx, 12
• Repe cmpsb repeat while sourcedest
• In instruction cmp ax, 5 (source 5 is subtracted
  from destination ax.
• In instruction cmps dest, source (dest is
  subtracted from the source)

9
SCASb Scan string byte

• Mov si, offset source
• Mov al, 20h
• Mov cx, 12
• Repne scasb repeat while sourcedest

10
SCASb Scan string byte

• Scan string backwards
• Mov si, offset source
• Mov al, 20h
• Mov cx, 12
• Std set the direction flag
• Repe scasb repeat while sourcedest

11
STOSb store string byte

• Mov di, offset str1
• Mov cx, 75
• Mov al, 0FFh
• Cld
• Rep stosb first 100 bytes of str1 initalized
  to FFh

12
LODSb load string byte

• loads al with one byte from a buffer
• Mov si, offset buffer
• Mov di, offset str1
• Mov cx, 10
• L1 lodsb
• And al, 7Fh
• Stosb
• Loop L1