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Direction Flag. one of 8086 processor control flags. controls the direction of string operations: DF = 0 => forward (left to right) processing; DF = 1 => backward (right to left) processing. CLD - clears the DF; sets DF = 0. STD - sets DF = 1
10 Mar 2012 In a nutshell, when the direction flag is 0, the instructions work by incrementing the pointer to the data after every iteration (until ECX is zero or some other condition, depending on the flavour of the REP prefix), while if the flag is 1, the pointer is decremented. STD SeTs the Direction flag, data goes backwards.
Some instructions generate exactly the same machine code, so disassembler may have a problem decoding to your original code. This is especially important for Conditional Jump instructions (see "Program Flow Control" in Tutorials for more information). Instructions in alphabetical order: Instruction. Operands. Description.
Thus, it is important that before we use the REP instruction prefix the following steps must be carried out. CX must be initialized to the count value. If auto decrementing is required, DF must be set using STD instruction else cleared using CLD instruction. E.g. MOV CX, 0023H CLD REP MOVSB. The above
To make DF = 1, use the std instruction. std ;set direction flag. cld and std have no effect on the other flags. Moving a String. Suppose we have defined two strings. DATASEG string1 DB "HELLO" string2 DB 5 DUP (?). The movsb instruction. movsb ;move string byte. copies the contents of the byte addressed by DS:SI to the
The MOV instruction is the most important command in the 8086 because it moves data from one location to another. . The 8086 uses a simple stack in memory for the storage of temporary data. .. If the direction flag is set instead of being cleared (using STD) then the 8086 will decrement SI and DI after each iteration.
To make DF = 1, use the std instruction. std ;set direction flag. cld and std have no effect on the other flags. Moving a String. Suppose we have defined two strings. DATASEG string1 DB "HELLO" string2 DB 5 DUP (?). The movsb instruction. movsb ;move string byte. copies the contents of the byte addressed by DS:SI to the
Set Direction Flag (std). std. Operation. 1 -> DF. Description. Sets the direction flag to 1, causing all subsequent string operations to decrement the index registers, (E)SI and/or (E)DI, used during the operation. Example. Set the direction flag: std. Previous: Clear Direction Flag (cld) · Next: Arithmetic Logical Instructions.
The direction flag is a flag that controls the left-to-right or right-to-left direction of string processing, stored in the FLAGS register on all x86-compatible CPUs. It is bit number 10. This flag is used to determine the direction (forward or backward) in which several bytes of data will be copied from one place in the memory,
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