1. Assembly 07
String Processing

2. Definition Defining strings Using sting length Zero Ending Explicitly
string DB ’Error message’
str_len DW 13
Relative
str_len DW $ - string
Zero Ending
string1 DB ’This is OK’,0
string2 DB ’Price = $9.99’,0

3. String instruction
String instruction may require a source operand, a destination operand, or both.
String instructions use ESI and EDI registers to point to the source and destination operands.
The source operand is assumed to be at ESI and the destination operand at EDI in memory.
For 16-bit segments SI and DI are used
The registers/indices are update automatically as the instruction is performed

4. Rep instruction Rep instruction unconditional repeat
rep REPeat
conditional repeat
repe REPeat while Equal
repz REPeat while Zero
repne REPeat while Not Equal
repnz REPeat while Not Zero
This instruction causes the instruction to repeat according to the value in ECX/CX
while (ECX = 0)
execute the string instruction;
ECX := ECX–1;
end while

5. Direction Flag
The direction of string operations depends on the value of the direction flag.
Direction flag (DF) is clear (DF = 0)
String operations proceed in the forward direction, from head to tail of a string.
Direction flag (DF) is set (DF = 1)
String processing is done in the opposite direction, from head to tail of a string.
DF instruction
std set direction flag (DF = 1)
cld clear direction flag (DF = 0)

6. String instructions String move Formats:
movs dest_string,source_string
movsb
movsw
Movsd
The suffix b, w, or d is used to indicate byte, word, or doubleword operands.
The source string value is pointed to by ESI and the destination string location is indicated by EDI in memory.
After copying, the ESI and EDI registers are updated according to the value of the direction flag.

7. Code Example .DATA string1 db ’The original string’,0
strLen EQU $ - string1
.UDATA
string2 resb 80
.CODE
.STARTUP
mov AX, DS ; set up ES
mov ES, AX ; to the data segment
mov ECX, strLen ; strLen includes NULL
mov ESI, string1
mov EDI, string2
cld ; forward direction
rep movsb

8. Load String Load a String (lods)
This instruction copies the value from the source string (pointed to by ESI) in memory to
AL (for byte operands—lodsb),AX (forword operands—lodsw), or EAX (for doubleword
operands—lodsd).
lodsb— load a byte string
AL := (ESI) ; copy a byte
if (DF = 0) ; forward direction
then
ESI := ESI+1
else ; backward direction
ESI := ESI−1
end if

9. Store a String (stos) stosb— store a byte string
This instruction performs the complementary operation. It copies the value in AL (for stosb), AX (for stosw), or EAX (for stosd) to the destination string (pointed to by ES:EDI) in
memory.
stosb— store a byte string
ES:EDI := AL ; copy a byte
if (DF = 0) ; forward direction
then
EDI := EDI+1
else ; backward direction
EDI := EDI−1
end if
Flags affected: none

10. Initializing an array Initializing array with −1. .UDATA
array1 resw 100
.CODE
.STARTUP
mov AX,DS ; set up ES
mov ES,AX ; to the data segment
mov ECX,100
mov EDI,array
mov AX,-1
cld ; forward direction
rep stosw

11. String Compare Instruction
The cmps instruction can be used to compare two strings. Like the cmp instruction, cmps performs ESI − EDI
cmpsb— compare two byte strings
Compare the two bytes at ESI and EDI and set flags
if (DF = 0) then ; forward direction
ESI := ESI+1
EDI := EDI+1
else ; backward direction
ESI := ESI−1
EDI := EDI−1
end if
Flags affected: As per cmp instruction

12. Using cmps
We can use conditional jumps like ja, jg, jc, etc. to test the relationship of the two values. As usual, the ESI and EDI registers are updated according to the value of the direction flag and the operand size.
The cmps instruction is typically used with the repe/repz or repne/repnz prefix.
.DATA
string1 db ’abcdfghi’,0
strLen EQU $ - string1
string2 db ’abcdefgh’,0
.CODE

13. Using cmps
.STARTUP mov AX,DS ; set up ES mov ES,AX ; to the data segment mov ECX,strLen mov ESI,string1 mov EDI,string2 cld ; forward direction repe cmpsb leaves ESI pointing to g in string1 and EDI to f in string2. Therefore, adding dec ESI dec EDI leaves ESI and EDI pointing to the last character that differs. Then we can use, ja str1Above

14. Scanning a String
The scas (scanning a string) instruction is useful in searching for a particular value or character in a string. The value should be in AL (for scasb), AX (for scasw), or EAX (for scasd), and ES:EDI should point to the string to be searched. scasb— scan a byte string Compare AL to the byte at ES:EDI and set flags if (DF = 0) then; forward direction EDI := EDI+1 else ; backward direction EDI := EDI−1 end if Flags affected: As per cmp instruction

15. Using scas .DATA string1 db ’abcdefgh’,0 strLen EQU $ - string1 .CODE
.STARTUP
mov AX,DS ; set up ES
mov ES,AX ; to the data segment
mov ECX,strLen
mov EDI,string1
mov AL,’e’ ; character to be searched
cld ; forward direction
repne scasb
dec EDI
This program leaves EDI pointing to e in string1. The following example can be used
to skip the initial blanks.

16. Example: Skip initial blanks
.DATA string1 db ’ abc’,0 strLen EQU $ - string1 .CODE .STARTUP mov AX,DS ; set up ES mov ES,AX ; to the data segment mov ECX,strLen mov EDI,string1 mov AL,’ ’ ; character to be searched cld ; forward direction repe scasb dec EDI