# Why is address space allocation granularity 64K?

You may have wondered why VirtualAlloc allocates memory at 64K boundaries even though page granularity is 4K.

You have the Alpha AXP processor to thank for that.

On the Alpha AXP, there is no "load 32-bit integer" instruction. To load a 32-bit integer, you actually load two 16-bit integers and combine them.

So if allocation granularity were finer than 64K, a DLL that got relocated in memory would require two fixups per relocatable address: one to the upper 16 bits and one to the lower 16 bits. And things get worse if this changes a carry or borrow between the two halves. (For example, moving an address 4K from 0x1234F000 to 0x12350000, this forces both the low and high parts of the address to change. Even though the amount of motion was far less than 64K, it still had an impact on the high part due to the carry.)

But wait, there's more.

The Alpha AXP actually combines two signed 16-bit integers to form a 32-bit integer. For example, to load the value 0x1234ABCD, you would first use the LDAH instruction to load the value 0x1235 into the high word of the destination register. Then you would use the LDA instruction to add the signed value -0x5433. (Since 0x5433 = 0x10000 - 0xABCD.) The result is then the desired value of 0x1234ABCD.

```LDAH t1, 0x1235(zero) // t1 = 0x12350000
LDA  t1, -0x5433(t1)  // t1 = t1 - 0x5433 = 0x1234ABCD
```

So if a relocation caused an address to move between the "lower half" of a 64K block and the "upper half", additional fixing-up would have to be done to ensure that the arithmetic for the top half of the address was adjusted properly. Since compilers like to reorder instructions, that LDAH instruction could be far, far away, so the relocation record for the bottom half would have to have some way of finding the matching top half.

What's more, the compiler is clever and if it needs to compute addresses for two variables that are in the same 64K region, it shares the LDAH instruction between them. If it were possible to relocate by a value that wasn't a multiple of 64K, then the compiler would no longer be able to do this optimization since it's possible that after the relocation, the two variables no longer belonged to the same 64K block.

Forcing memory allocations at 64K granularity solves all these problems.

If you have been paying really close attention, you'd have seen that this also explains why there is a 64K "no man's land" near the 2GB boundary. Consider the method for computing the value 0x7FFFABCD: Since the lower 16 bits are in the upper half of the 64K range, the value needs to be computed by subtraction rather than addition. The naïve solution would be to use

```LDAH t1, 0x8000(zero) // t1 = 0x80000000, right?
LDA  t1, -0x5433(t1)  // t1 = t1 - 0x5433 = 0x7FFFABCD, right?
```

Except that this doesn't work. The Alpha AXP is a 64-bit processor, and 0x8000 does not fit in a 16-bit signed integer, so you have to use -0x8000, a negative number. What actually happens is

```LDAH t1, -0x8000(zero) // t1 = 0xFFFFFFFF`80000000
LDA  t1, -0x5433(t1)   // t1 = t1 - 0x5433 = 0xFFFFFFFF`7FFFABCD
```

You need to add a third instruction to clear the high 32 bits. The clever trick for this is to add zero and tell the processor to treat the result as a 32-bit integer and sign-extend it to 64 bits.

```ADDL t1, zero, t1    // t1 = t1 + 0, with L suffix
// L suffix means sign extend result from 32 bits to 64
// t1 = 0x00000000`7FFFABCD
```

If addresses within 64K of the 2GB boundary were permitted, then every memory address computation would have to insert that third ADDL instruction just in case the address got relocated to the "danger zone" near the 2GB boundary.

This was an awfully high price to pay to get access to that last 64K of address space (a 50% performance penalty for all address computations to protect against a case that in practice would never happen), so roping off that area as permanently invalid was a more prudent choice.

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1. Jordan Russell says:

Very interesting. But why, though, do the x86 versions of Windows have to use 64K granularity? Is it just for consistency?

2. Peter Lund says:

Couldn’t that have been solved by using a global offset table and a global pointer like they did in Digital Unix (OSF/1) ?

Instead of constructing 32-bit values piece by piece with lda/ldah (that adds the 16-bit immediate field in the 32-bit instruction to a register) you could "just" load it from a table in memory.

3. Raymond Chen says:

Yes, it could’ve been done that way, and in fact the MIPS did work that way. MIPS support actually predated Alpha support, and the 64K granularity existed even on the MIPS but for different reasons. (Or so I’ve been told by people who were alive back then.)

The granularity exists even on the x86 because NT is a single code base for all architectures. You don’t want to introduce gratuitous differences between architectures because it makes porting code between them harder. (Somebody will write code on x86 that assumes 4K granularity and run it on an Alpha and it will crash – or vice versa – and then they go nuts trying to figure out what went wrong, until they stumble across a small sentence buried somewhere in MSDN and then curse Microsoft for making gratuitous differences between their platforms.)

4. You’ve run out of address space.

5. Preserving the spirit while accommodating separate address spaces and new processors.

6. Belated answers to exercises and other questions.

7. Beats me.