3 ms·

Slightly related (but only slightly), on the PS2 (games console, from Sony) you could access main memory with three different pointers, cacheable, uncached acce

by scraft 8y ago

Slightly related (but only slightly), on the PS2 (games console, from Sony) you could access main memory with three different pointers, cacheable, uncached accelerated and uncached (which was the same base address with different high bits set accordingly). Each mode had reasons to be used (for example, when you read/write to memory which is cacheable and then fire off a DMA transfer, the data being transferred may, and often won't, match, what has been set in the program, until a cache flush has been requested). As you can imagine, this regularly resulted in hard to track down bugs. Anyway, the bottom line of this is: I have seen various pieces of code on platforms which when comparing a memory pointer you need to mask out several bits (unless of course you actually want to differentiate between the different access methods of the same piece of memory). It isn't quite what this article is talking about, but I thought it was a related piece of interesting trivia to those that don't already know ;)

karmakaze 8y ago
Great example to add to the discussion. The post does mention that pointers may have some meta bits which I fully accept but without concrete examples wouldn't give much consideration for. Reminds me of the recent underspec that was closed with signed/unsigned some operation or other. Better for the spec but can't imagine practically mattered. If some unbelievable situation occurs on some unique platform/compiler remember this post. In the meantime do as we have been with using pointer values. Back before the System7 32bit clean code people put all sorts of stuff in pointers because we could. We know the costs but also remember that it had value too.
mrslave 8y ago
Smells like a MIPS processor? I really enjoyed this stuff at university. I work with very high level tools these days, informing business decision makers, and I don't regret the move up the value stack. In fact I have found that being a competent developer at low levels (embedded systems, operating systems) has helped solve some incredibly subtle issues with high-level tools in my career.
derefr 8y ago
> which was the same base address with different high bits set accordingly Reminds me of memory mirroring on the NES (https://wiki.nesdev.com/w/index.php/Mirroring#Memory_Mirroring https://wiki.nesdev.com/w/index.php/Mirroring#Memory_Mirrori...). Except that, in this case, the mirrored memory has different access semantics :)

by scraft 8y ago

karmakaze 8y ago

Great example to add to the discussion. The post does mention that pointers may have some meta bits which I fully accept but without concrete examples wouldn't give much consideration for. Reminds me of the recent underspec that was closed with signed/unsigned some operation or other. Better for the spec but can't imagine practically mattered. If some unbelievable situation occurs on some unique platform/compiler remember this post. In the meantime do as we have been with using pointer values. Back before the System7 32bit clean code people put all sorts of stuff in pointers because we could. We know the costs but also remember that it had value too.

mrslave 8y ago

Smells like a MIPS processor? I really enjoyed this stuff at university. I work with very high level tools these days, informing business decision makers, and I don't regret the move up the value stack. In fact I have found that being a competent developer at low levels (embedded systems, operating systems) has helped solve some incredibly subtle issues with high-level tools in my career.

derefr 8y ago

> which was the same base address with different high bits set accordingly Reminds me of memory mirroring on the NES (https://wiki.nesdev.com/w/index.php/Mirroring#Memory_Mirroring https://wiki.nesdev.com/w/index.php/Mirroring#Memory_Mirrori...). Except that, in this case, the mirrored memory has different access semantics :)