Use x86 String Instructions (MOVSB, STOSB, LODSB)

String instructions

LODSB loads from DS:SI, STOSB stores to ES:DI, MOVSB copies byte from DS:SI to ES:DI, each optionally repeating with REP using CX as a counter. For example, REP MOVSB copies 80 bytes from a source buffer to 0xB8000 for a fast screen fill when direction flag is cleared.

Direction and prefixes

nasmCopy
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• STD: Set direction flag for backward copies (overlap caution)
• REPNE/REPE: Used with SCASB/CMPSB for strlen and strcmp
• Segment overrides: ES is fixed for STOS/MOVS destination
• String ops are compact compared to hand-rolled loops in boot sectors

Boot-sector sizing

String instructions shrink loop code dramatically inside the 510-byte budget of a boot sector. Always issue CLD before forward REP MOVSB unless you intentionally copy backward for overlapping regions. Remember that ES cannot be the same arbitrary segment as DS unless you load it; many snippets set both to the same paragraph for video memory copies to 0xB800.

Why for this exercise

You will implement memory fill and copy with REP STOSB and REP MOVSB. This exercise requires setting DS, ES, SI, DI, and CX correctly and clearing the direction flag before forward copies.