String Instructions

1
String Instructions

• Assembly Language Programming
• University of Akron
• Dr. Tim Margush

2
String?

• An array of bytes or words located in memory
• Supported String Operations
• Copy
• Search
• Store
• Compare

3
String Instruction Basics

• Source DSSI, Destination ESDI
• You must ensure DS and ES are correct
• You must ensure SI and DI are offsets into DS and
  ES respectively
• Direction Flag (0 Up, 1 Down)
• CLD - Increment addresses (left to right)
• STD - Decrement addresses (right to left)

4
Moving (Copying)

• MOVSB, MOVSW
• Memory to memory copy of a byte or word
• Each execution of this instruction causes
• Copy byte/word at DSSI to ESDI
• Inc/Dec SI and DI by 1 or 2
• If CX contains a repetition factor
• REP MOVSB or REP MOVSW will automatically execute
  the move CX times, and CX becomes 0

5
Example Copy a String

• Copy array a to b, assume ESDS, and 10 bytes
  are to be copied
• mov cx, 10 10 bytes to copy
• mov di, offset b destination
• mov si, offset a source cld left to right
• rep movsb

6
Example Memory Shift

• shift bytes of a 3 bytes to right
• mov cx, 7 bytes to copy
• mov di, offset a9 dest
• mov si, offset a9-3 source
• std copy from right to left
• rep movsb

SI
DI
a
7
Example Replication

• pattern db "!_at_" duplicate
• db (100-4) dup (?) space
• mov cx,100-4 96 bytes to copy mov si, offset
  pattern
• mov di, offset pattern4
• cld destructive overlap
• rep movsb

DI
SI
!
_at_


a
8
Store String

• STOSB, STOSW
• Copy AL or AX into an array of bytes or words
• destination ESDI
• Each repetition Increments or Decrements DI
• depends on DF

• Commonly used with REP prefix and number of
  repetitions in CX
• The Word version byte reverses AX as usual

9
Example Initilializing Storage

• arr dw 200 dup (?) empty words
• to be initialized to A050A050...
• mov ax,50A0h
• mov di,offset arr
• mov cx,200 array size
• cld
• stosw

DI
50
A0
AX
A0
50
A0
50
arr
10
Load String

• LODSB, LODSW
• Byte or word at DSSI is copied into AL or AX
• SI is increased or decreased by 1 or 2
• This is commonly paired with STOSx in a loop to
  process each component of an array
• There is no reason to use REP with this
  instruction

11
Example Process Array

• array b toUpper(array a)
• mov di, offset b dest
• mov si, offset a source
• mov cx,30 array size cld left to right
  processing
• lp
• lodsb get next byte
• and al,0DFh to upper case
• stosb store at next location
• loop lp

12
Scan String

• SCASB, SCASW
• Compares AL or AX with ESDI and auto increments
  or decrements DI
• This instruction sets the flags register
• Flags set according to result of compare
• Used in a loop, or with conditional REPs
• REPZ, REPE, REPNZ, REPNE

13
Conditional Repeats for SCASx and CMPSx

• while (CX ! 0 )
• do string primitive
• --CX
• if (REPNE and ZF 1) exit loop
• if (REPE and ZF 0) exit loop

• The test for CX is at the top of the loop
• Test of zero flag is at the end of the loop.
• Only CMPS and SCAS instructions can affect the ZF
  value

14
Example String Search

• arr db 'abcdefghijklmnopqrstuvwxyz'
• mov di, offset arr
• mov cx,26 26 bytes
• cld left to right processing
• mov al,target ch to find
• repne scasb search for match
• make at most cx comparisons
• jne nomatch ZF never set
• match occurred at ESdi-1
• di is incremented even if match

15
Compare String

• CMPSB, CMPSW
• Compares DSSI to ESDI, setting flags and
  auto-increments/decrements SI and DI
• Used to compare arrays of words or arrays of
  bytes
• Typically used with conditional REP instruction

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Example String Compare

• mov si, offset str1
• mov di, offset str2
• cld left to right processing
• mov cx, 12 shorter string
• repe cmpsb cmp til ltgt or cx0
• jl str1smaller
• jg str2smaller
• the strings are equal - so far
• if sizes different, shorter string is less

17
Homework

• Pg 225
• 1-4