2018年系統軟體短期課程: 隨堂測驗

Q1: Unaligned memory access

An unaligned memory access is a load/store that is trying to access an address location which is not aligned to the access size.

e.g: A load instruction trying to access 4 bytes from address 0x1 is an unaligned access. This typically gets split into two internal operations as shows in the following diagram and merges into one.

Note that in case of systems with caches, there can also be cases of addresses crossing a cache line boundary (a.k.a misaligned access) and some times accesses can also be crossing a page boundary.

Then you can also have other complexities like one half of the memory access is hit in cache while the other half is a miss in the cache. The load store execution unit along with cache controllers deals with these complexities but in summary it follows the same basic principle of merging two access

Some architectures (like Intel x86) also has alignment interrupts that help in detecting unaligned memory access.

考慮以下 unaligned_get32 函式的實作: (假設硬體架構為 32-bits)

#include <stdint.h>
#include <stddef.h>
uint8_t unaligned_get8(void *src) {
    uintptr_t csrc = (uintptr_t) src;
    uint32_t v = *(uint32_t *) (csrc & 0xfffffffc);
    v = (v >> (((uint32_t) csrc & 0x3) * 8)) & 0x000000ff;
    return v;
}
uint32_t unaligned_get32(void *src) {
    uint32_t d = 0;
    uintptr_t csrc = (uintptr_t) src;
    for (int n = 0; n < 4; n++) {
        uint32_t v = unaligned_get8((void *) csrc);
        v = v << (n * 8);
        d = d | v;
        csrc++;
    }
    return d;
}

「補完」對應的 unaligned_set8 函式:

void unaligned_set8(void *dest, uint8_t value) {
    uintptr_t cdest = (uintptr_t) dest;
    uint32_t d = *(uint32_t *) (cdest & 0xfffffffc);
    uint32_t v = value;
    for (int n = 0; n < 4; n++) {
        uint32_t v = unaligned_get8((void *) csrc);
        v = v << (n * 8);
        d = d | v;
        csrc++;
    }
    return d;
}
void unaligned_set32(void *dest, uint32_t value) {
    uintptr_t cdest = (uintptr_t) dest;
    for (int n = 0; n < 4; n++) {
        unaligned_set8((void *) cdest, value & 0x000000ff);
        value = value >> 8;
        cdest++;
    }
}

參考資料: