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0 pointslelanthran5d ago0 comments

> A zero is a sentinel value and is catered to by all ISAs.

> Why would using a "$" be any easier/safer than a NUL?

I didn't say it had to be '$'; I specifically said that the sentinel would be loaded into a register. In that case it could be anything, including zero (for the snippet you posted), or INT_MAX if the code iterated across an array of integers, etc.

By having rep/mov variants that use sentinels, a lot of the HLL problems go away - Java, C#, Python, etc would all look very different today if the ISAs from the 80s included sentinal variant of memory instructions.

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rswail5d ago· 2 in thread

Except that nearly all ISAs treat zero as a special value, with a Z-flag or equivalent for the last ALU result, and conditional branches around that result.

PDP-11s, 68Ks, nearly all ISAs that I know about treat zero as special.

It falls naturally out of the ALU operations.

So why would people writing assembler code use another value unless they had to?

lelanthranOP5d ago

That's my point - they didn't, and used the zero as a sentinel when designing their HLL.

If, OTOH, the ISA had additional variants of those instructions that allowed usage of anything as a sentinel, HLL implementations of array would never have needed a fat pointer (length + memory).

rswail4d ago

Except that the ISA has a perfectly good ALU there that can detect zero really easily, so no one was going to waste silicon on an instruction that required comparison to yet another value (which essentially would be an additional subtraction or OR or equivalent) added to the loop.

The fat pointers are much more efficient in that you don't need to scan memory to get the length or find the end to append or take slices etc.

Especially for vectors that don't have any value that can be used as a sentinel.

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