16bit Memory Code

Clearing or Setting a Block of Memory

There will be times when you need to set the contents of a memory block to a default value.

The following code peforms this action quickly.

; Clears a block of memory ; Pass EDI = Destination Address ; ECX = Number of bytes to write ; EAX = AL holds the character to be written SubMemoryClear: PUSH EDI PUSH ECX STOSB NOP ; The No operation is necessary with some processors LOOP 0xFC POP ECX POP EDI RET

Moving a Block of Memory

This routine is about as fast as it gets in 16bit. It you want to move a null rerminate string without knowing its length use the string routine that follows.

It will also copy to and from other segments if you set the segment pointers before calling.

; Copies a block of memory regardless of content ; Pass EDI = Destination address ; ESI = Source Address ; ECX = number of characters to be copied ; It ignores the NULL terminator SubMemoryBlockCopy: PUSH EAX PUSH EDI PUSH ESI PUSH ECX CLD LODSB STOSB LOOP 0xFC POP ECX POP ESI POP EDI POP EAX RET

Moving a String in Memory

This routine is about as fast as it gets in 16bit. It you want to move a null rerminate string without knowing its length use the string routine that follows.

The routine can also be used to copy the left part of a string, in which case you must remember to add a NULL at the end.

; Copies a string in memory, stops when either a NULL is reached or the ; Length stated in CX. Copies the NULL if it exists before exiting. ; Pass EDI = Destination address ; ESI = Source Address ; ECX = Maximum number of characters to be copied SubMemoryStringCopy: PUSH EAX PUSH EDI PUSH ESI PUSH ECX CLD LODSB STOSB CMP AL, 0x00 ; Check for the NULL if present JE 0x02 ; Jump out of loop LOOP 0xFC POP ECX POP ESI POP EDI POP EAX RET

Last Update 27/12/2022