Need Help With Quick Bounds Check

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Hayden

Joined: 06 Oct 2005
Posts: 132

Hayden 07 Nov 2007, 09:33

I'ts a pretty dumb but I hope someone could save me some time. I want to take a memory address and calculate the upper and lower limits of that page.

so for memory mapping with 64k pages the address 12345 yields {0,65535} and address 98302 would yield {65536,131070}.

I know i can use MOD but i'm looking for the quickest and easeist way. ps. i can not assume pages to be 64k.

_________________
New User.. Hayden McKay.

07 Nov 2007, 09:33

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4623
Location: Argentina

LocoDelAssembly 07 Nov 2007, 15:33

Code:

PAGE_SIZE = 65536 ; Always 2^n where n can go from 0 to 30
MOD_MASK = -PAGE_SIZE

calculate_bounds:; EAX = Address
mov edx, PAGE_SIZE-1
and eax, MOD_MASK
add edx, eax

ret ; EAX = lower limit; EDX = upper limit

[edit] Or maybe better

Code:

calculate_bounds:; EAX = Address 
and eax, MOD_MASK
lea edx, [eax+PAGE_SIZE-1]

ret ; EAX = lower limit; EDX = upper limit

[/edit]

07 Nov 2007, 15:33

bitRAKE

Joined: 21 Jul 2003
Posts: 4311
Location: vpcmpistri

bitRAKE 07 Nov 2007, 18:11

; assuming granularity is a power of two
add eax, granularity-1
and eax, 0-granularity
; EAX is upper bound (beyond page)
; just subtract page size to get lower bound

07 Nov 2007, 18:11

Hayden

Joined: 06 Oct 2005
Posts: 132

Hayden 08 Nov 2007, 08:53

thanks guys, the code i was useing was like this
persudo code...

UpperBounds = (address + grans) - (address mod grans)
LowerBounds = address - (address mod grans)

_________________
New User.. Hayden McKay.

08 Nov 2007, 08:53

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