The Alpha AXP, epilogue: A correction about file system compression on the Alpha AXP

Raymond Chen August 31, 2017 Aug 31, 2017 08/31/17

Some time ago, I claimed that Windows file compression had to be dumbed down in order for the Alpha AXP to hit its performance targets. I have since been informed by somebody who worked on file system compression for the Windows NT that information was badly sourced. (My source was somebody who worked on real-time file compression, but not ...

The Alpha AXP, part 17: Reconstructing a call stack

Raymond Chen August 30, 2017 Aug 30, 2017 08/30/17

I'm going to wrap up the formal part of the series by applying the information we've been learning over the past several days weeks: We're going to reconstruct a broken stack. Suppose you have a debug session where all the k command says is That's it. We'll have to unwind the stack manually. We need to keep track of the stack pointer depth ...

The Alpha AXP, part 16: What are the dire consequences of having 32-bit values in non-canonical form?

Raymond Chen August 29, 2017 Aug 29, 2017 08/29/17

On the Alpha AXP, 32-bit values are typically represented in so-called canonical form. But what happens if you use a non-canonical representation? Well, it depends on what instruction consumes the non-canonical representation. If the consuming instruction is an explicit 32-bit instruction, such as ADDL or STL, then the upper 32 bits are ...

The Alpha AXP, part 15: Variadic functions

Raymond Chen August 28, 2017 Aug 28, 2017 08/28/17

As noted in the initial plunge, the first six integer parameters are passed in registers, and the first six floating point parameters are passed in a different set of registers. So how does the callee known at function entry which registers to spill, and in what order?¹ Answer: It doesn't. So it just spills everything. First, a detail ...

The Alpha AXP, part 14: On the strange behavior of writes to the zero register

Raymond Chen August 25, 2017 Aug 25, 2017 08/25/17

I noted early on that a special rule in the Alpha AXP is that an instruction that specifies the zero register as a destination is permitted to be completely optimized out by the processor (with two exception, as noted). You might wonder what the point of this rule is. I mean, if you want the processor to not execute an instruction, then just ...

The Alpha AXP, part 13: On treating a 64-bit processor as if it were a 32-bit processor

Raymond Chen August 23, 2017 Aug 23, 2017 08/23/17

The Alpha AXP is natively a 64-bit processor, but Windows NT for Alpha AXP treated it as a 32-bit processor. How is that possible? The Alpha AXP registers are full 64-bit registers, and you are welcome to use all 64 bits to perform your computations, in the same way that you are welcome to use the 32-bit registers of the x86 even when the ...

The Alpha AXP: Part 12: How you detect carry on a processor with no carry?

Raymond Chen August 22, 2017 Aug 22, 2017 08/22/17

The Alpha AXP has no corresponding trap variant for arithmetic carry. So how would you detect carry?¹ Answer: The same way you detect carry in C, or pretty much any other programming language that doesn't support carry. To detect carry during addition, you check whether the sum is less than either addend. If the sum is less than one ...

The Alpha AXP, part 11: Processor faults

Raymond Chen August 21, 2017 Aug 21, 2017 08/21/17

There are three types of faults on the Alpha AXP: Software faults are those triggered by explicit instructions, such as CALL_PAL. These are calls that trap into the kernel and are used as the Alpha AXP version of syscall. Software faults are raised synchronously, and execution does not proceed past a software fault. Consequently, they are...

The Alpha AXP, part 10: Atomic updates to byte and word memory units

Raymond Chen August 18, 2017 Aug 18, 2017 08/18/17

Today we're going to do a little exercise based on what we've learned so far. We learned how to perform byte and word loads and stores to memory. And we also learned how to perform atomic memory operations on longs and quads. But how about atomic memory operations on bytes and words? We will have to put together what we've learned: Combine ...