## ALP Programming Guide and Reference

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### Description

The STOS instruction transfers the contents of the AL, AX, or EAX register to the memory byte or word given by the destination register relative to the ES segment. The destination register is the DI register for an address- size attribute of 16 bits or the EDI register for an address-size attribute of 32 bits.

The destination operand must be addressable from the ES register. A segment override is not possible.

The address of the destination is determined by the contents of the destination register, not by the explicit operand of the STOS instruction. This operand is used only to validate ES segment addressability and to determine the data type. Load the correct index value into the destination register before running the STOS instruction.

After the transfer is made, the (E)DI register is automatically updated. If the DF flag is 0 (the CLD instruction was run), the (E)DI register is incremented; if the DF flag is 1 (the STD instruction was executed), the (E )DI register is decremented. The (E)DI register is incremented or decremented by 1 if a byte is stored, by 2 if a word is stored, or by 4 if a doubleword is stored.

The STOSB, STOSW, and STOSD instructions are synonyms for the byte, word, and doubleword STOS instructions, that do not require an operand. They are simpler to use, but provide no type or segment checking.

The STOS instruction can be preceded by the REP prefix for a block fill of CX or ECX bytes, words, or doublewords. Refer to the REP instruction for further details.