How does the .bss get cleared?

[connorkuehl]

Does x1 get incremented somehow? I think x1 has the same value throughout the loop so it looks like with every iteration a zero is being written to the same place.

In start.S:

    // clear bss
    ldr     x1, =__bss_start
    ldr     w2, =__bss_size
3:  cbz     w2, 4f
    str     xzr, [x1], #8
    sub     w2, w2, #1
    cbnz    w2, 3b

I'm also curious about the immediate offset in the str instruction. Is it necessary?

[connorkuehl]

Actually, after stepping through it with gdb it would appear the str instruction can update the base register. See the "Index Modes" sub section in the ARM Cortex-A Series Programmer's Guide for ARMv8

[bztsrc]

Hi,

That's right. The #8 constant in the str instruction tells the CPU to adjust the offset register by 8 bytes.

Cheers,
bzt