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.

When it is set to 0 (using the clear-direction-flag instruction CLD),^[3] it means that instructions that autoincrement the source index and destination index (like MOVS) will increase both of them. In case it is set to 1 (using the set-direction-flag instruction STD),^[4] the instruction will decrease them.

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.

References

x86 assembly topics

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

x86 Assemblers	A86/A386 FASM GAS HLA MASM NASM TASM WASM YASM

Programming issues	Call stack Flags Carry flag Direction flag Interrupt flag Overflow flag Zero flag Opcode Program counter Processor register x86 calling conventions x86 instruction listings x86 registers