PAk-Aims Lecture

1. 1
Strings

2. 2
String Primitive Instructions, Implied Operands.
Instruction Description ImpliedOperands
MOVS
MoveStringData: Copyamemory
byte, word, ordoublewordfromone
memorylocationtoanother.
MOVS [DI],[SI]
CMPS
CompareString: Compareamemory
byte, word, ordoublewordto
memory.
CMPS [SI],[DI]
SCAS
ScanString: CompareAL, AX, or
EAXtothecontentsofmemory,
affectingtheFlagsregister.
SCAS [DI],AL
SCAS [DI],AX
SCAS [DI],EAX
STOS StoreStringData: StoreAL, AX, or
EAXintomemory.
STOS [DI],AL
STOS [DI],AX
STOS [DI],EAX
LODS
LoadAccumulatorfromString:
Loadabyte, word, ordoubleword
intoAL, AX, orEAXfrommemory.
LODS AL,[SI]
LODS AX,[SI]
LODS EAX,[SI]

3. 3
Overview of String Primitive Instructions.
General Specific Size Description
Increment Value
for SI and DI
MOVS MOVSB Move (copy) byte 1
MOVSW Move (copy) word 2
MOVSD Move (copy) doubleword* 4
CMPS CMPSB Compare bytes 1
CMPSW Compare words 2
CMPSD Compare doublewords* 4
SCAS SCASB Scan byte 1
SCASW Scan word 2
SCASD Scan doubleword* 4
STOS STOSB Store byte 1
STOSW Store word 2
STOSD Store doubleword* 4
LODS LODSB Load byte 1
LODSW Load word 2
LODSD Load doubleword* 4

4. 4
ValueoftheDirectionFlagEffectonSIandDIAddressSequence
0 Incremented Low-high
1 Decremented High-low
Direction Flags.

5. 5
MOVS (Move String Data).
Instruction
Valuetobeautomatically
added/subtractedfromSIandDI
MOVSB 1
MOVSW 2
MOVSD 4

6. 6
Unsigned and Signed Jumps.
Condition Unsigned Signed
source< dest JB JL
source<=dest JBE JLE
source≠dest JNE(JNZ) JNE(JNZ)
source= dest JE(JZ) JE(JZ)
source>=dest JAE JGE
source> dest JA JG

7. 7
Scan for a Matching Character.
.data
alpha db ‘ABCDEFGH’,0
.code
mov di,seg alpha
mov es,di
mov di,offset alpha ; ES:DI points to the string
mov al,’F’ ; search for the letter ‘F’
mov cx,8 ; set the search count
cld ; direction = up
repne scasb ; repeat while not equal
jnz exit ; quit if letter is not found
dec di ; found: back up DI one
character

8. 8
String-Handling Procedures.
Procedure Description
Str_compare
Compare two strings according to their
collating sequence. CF = 1 if the first
string is lesser, ZF = 1 if both strings
are equal, and (CF = 0, ZF = 0) if the
first string is greater.
Str_copy Copy a source string to a destination
string.
Str_length Find the length of a string.
Str_getline Read a null-terminated string froma file
or device.
Str_read Read a string froma file or device.
Str_ucase Convert a string to uppercase.
Str_write Write a string to a file or device.

9. 9
Flag Usage by the Str_compare Procedure.
RelationBetween
Strings
CarryFlag ZeroFlag
RelatedJump
Instruction
first<second 1 0 JB
first=second 0 1 JE
first>second 0 0 JA

10. 9
Flag Usage by the Str_compare Procedure.
RelationBetween
Strings
CarryFlag ZeroFlag
RelatedJump
Instruction
first<second 1 0 JB
first=second 0 1 JE
first>second 0 0 JA