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explaination on the power of two stuff
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Elaina Claus
2025-09-05 20:54:29 -04:00
parent 130cd22ff1
commit 69223a1ad2
1 changed files with 12 additions and 0 deletions
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include/util/arena_alloc.nasm
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@@ -54,6 +54,18 @@ arena_align_up:
jz .endp jz .endp
; enforce power-of-two for alignment, return x ; enforce power-of-two for alignment, return x
; for example...
; alignment = 0x0010,
; 0x0010 & (0x0010 - 0x0001)
; 0x0010 & 0x000F -> 1_0000000b & 0_11111111b => 0b == zero
;
; alignment = 0x0006
; 0x0006 & (0x0006 - 0x0001)
; 0x0006 & 0x0005 -> 00000110b & 00000101b => 00000100b != 0
;
; i.e any power of 2 has only 1 bit set in binary (2 = 0010b, 4 = 0100b, 8 = 1000b and so on)
; subtracting 1 from a power of 2 'flips' lower bits ( 7 = 0111b, 15 = 1111b ...)
; so 'and'ing a power of two with (itself - 1) will result in none of the bit's being set
mov cx, bx mov cx, bx
dec cx dec cx
test bx, cx test bx, cx