Direction flag

The direction flag is a flag that controls the left-to-right or right-to-left direction of string processing,^[1] stored in the FLAGS register on all x86-compatible CPUs.^[2] 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, to another. The direction is important mainly when the original data position in memory and the target data position overlap.

If it is set to 0 (using the clear-direction-flag instruction CLD) — it means that string is processed beginning from lowest to highest address; such instructions mode is called auto-incrementing mode. Both the source index and destination index (like MOVS) will increase them;
In case it is set to 1 (using the set-direction-flag instruction STD) — the string is processed from highest to lowest address.This is called auto-decrementing mode.

x86-instruction	Meaning	Flag	Notes
x86-instruction	Meaning	Flag	Direction of string processing	Mode title
CLD	clear direction flag	0	lowest-to-highest address	auto-incrementing
STD	set direction flag	1	highest-to-lowest address	auto-decrementing

References

x86 assembly topics

Topics	Assembly language Comparison of assemblers Disassembler Instruction set Low-level programming language Machine code Microassembler x86 assembly language

Assemblers	A86/A386 FASM GNU Assembler (GAS) High Level Assembly (HLA) Microsoft Macro Assembler (MASM) Netwide Assembler (NASM) Turbo Assembler (TASM) Open Watcom Assembler (WASM) Yasm

Programming issues	Call stack Flags Carry flag Direction flag Interrupt flag Overflow flag Zero flag Opcode Program counter Processor register Calling conventions Instruction listings Registers

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