As an academic exercise, two assembler alternatives attached as text files.

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MEMSET.txt uses STC/MVC to set 256 bytes at a time and then STC and variable-length MVC to set the remainder less than 256 bytes, with optimizations not to set anything unneeded if N is zero or an integer multiple of 256.

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MEMSET16.txt uses 2 more registers than MEMSET.txt but replaces the length=255 MVC instruction with multiple STore operations (loop 16 times storing 4 bytes at a time 4 times for each loop around).? MVC was notoriously slow on some real-iron IBM hardware models.

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Not sure which technique would have been faster on any real 360-era iron, but there could be differences in the Hercules MVC vs STore operations that may make the STore solution faster (or not).

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These are untested, so I could have errors in my coding.? Corrections and improvements welcome.

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A C solution doing the same sort of thing as MEMSET16.txt using casts to INT and taking advantage of the C compiler’s optimization and code generation could be even faster.? Something along these lines doing 16 bytes at a time (could obviously do 32 or 64 each loop as well, but 16 gives you the picture):

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void *memset(void *s, int c, size_t n)

{

? ? size_t x;

??? int cccc = c + (c << 8) + (c << 16) + (c << 24);

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? ? for (x = 0; x < (n / 16); x+=16)

? ? {

? ? ? ? *((int *)((char *)s + x???? )) = cccc;

? ? ? ? *((int *)((char *)s + x + ?4)) = cccc;

? ? ? ? *((int *)((char *)s + x + ?8)) = cccc;

? ? ? ? *((int *)((char *)s + x + 12)) = cccc;

? ? }

? ? for (; x < n; x++)

? ? {

? ? ? ? *((char *)s + x) = (unsigned char)c;

? ? }

? return (s);

}

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HTH

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Peter

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