Agner`s CPU blog

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Proposal now published
Author: Agner Date: 2016-03-24 01:46
Hubert Lamontagne wrote:
- The user unexpectedly loads a 300mb file in program 2, which causes a surprise 300mb allocation on the heap. Where does the OS place this allocation?
The heap doesn't have to contiguous - the stack does. In case the heap overflows, there are three options:
1. Allocate more heap space somewhere else. Make an extra entry in the memory map for it.
2. Same as 1. Use virtual address translation to keep it contiguous in order to make the heap manager simpler. This requires that you allocate a lot of unused virtual address space for each program as it starts. This method works also for the stack.
3. As a last resort, when memory space has become hopelessly fragmented and you are out of memory map entries. Swap the data of the least used program to disk. Reorganize the fragmented data by actually moving the values to make them contiguous in physical memory (assuming that they were already contiguous in virtual address space). This will cause an annoying delay to the user, but we already have such delays in current systems when you run out of memory. You may signal a warning to the user: Memory low, please close some programs.

One suggestion: remove "Indexing into the register file".
Thank you for pointing out the difficulties here. Several people have proposed things like a reconfigurable register space with arbitrary allocation of vectors in this space, and I thought that a register index was simpler.

I want to avoid complex instructions for saving and restoring all or many registers. Any suggestions for an alternative to writing 64 consecutive save instructions or having a complex microcoded save-all instruction?

2nd suggestion: Make operand size for tiny format 32bits, not 64bits (except mov). Consider trading tiny setbit/clearbit/xor/reversesubtract/andnot for arithmetic shift right and 64bit add/sub. Consider adding the option for 32bit signed indexes in address calculations or sign extending instead of zero extending 32bit operations to 64bit by default.
I am not sure if you are trying to save power by doing a 32-bit addition instead of 64 bits. The hardware may actually disable the upper part of the heavy carry-lookahead circuit if it can detect quickly that the values are small. Otherwise, I don't see the problem of using 64-bit addition on 32-bit values. The value will simply be truncated if it is later used in a 32-bit (not-tiny) instruction. The compiler will recognize the need to sign-extend a 32-bit signed index or optimize the program by replacing it with a 64-bit index variable. Or you may use an unsigned 32-bit index and use the carry flag for detecting end of loop if the index is counting down. I would rather keep array indexes 64-bit because setting a 2 GB size limit to arrays is a problem for the programming language standard, and the value may cross zero when a count-down loop ends.
 
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