1. CS-401 Computer Architecture & Assembly Language Programming
Lecture-18
String Instructions

2. Lets revise the last lecture

3. Multiply Instruction

4. String Instructions

5. stosb/stosw Store String
Transfer a byte/word from register AL/AX to the string element addressed by ES:DI and updates DI accordingly to point to the next location.
DF = 0 increment DI
DF = 1 decrement DI
Flags Affected: NONE
Uses:
Fill a constant in a block of memory.

6. lodsb/lodsw Load String
Transfer a byte/word from the source location DS:DI to location AL/AX. SI is updated accordingly to point to the next location.
Flags Affected: NONE
Normally rep prefix is not used with lods.

7. scansb/scansw Scan String
Compares the source byte/word in register AL/AX. With the destination location ES:DI and updates the flags. DI is updated accordingly to point to the next location.
Nothing changes except flags and DI.
Flags Affected: A C O P S Z
Uses:
Locate a byte / Skip certain byte

8. movsb/movsw Move String
Transfers a byte/word from a source location DI:SI to the destination location ES:DI and updates SI and DI accordingly to point to the next location.
Flags Affected: NONE
Uses:
Move block of memory

9. cmpsb/cmpsw Compare String
Subtracts the destination location ES:DI byte/word from source location DS:SI. Source and destination are unaffected. SI and DI are updated accordingly to point to the next location.
Flags Affected: A C O P S Z
Uses:
Search a Substring