40 lines
973 B
ArmAsm
40 lines
973 B
ArmAsm
/*
|
|
* Copyright (C) 2010 Apple Inc. All rights reserved.
|
|
*
|
|
* This document is the property of Apple Inc.
|
|
* It is considered confidential and proprietary.
|
|
*
|
|
* This document may not be reproduced or transmitted in any form,
|
|
* in whole or in part, without the express written permission of
|
|
* Apple Inc.
|
|
*/
|
|
#include <arch/arm/assembler.h>
|
|
|
|
.text
|
|
|
|
// u_int32_t arch_get_entropy(volatile u_int32_t *counter_address)
|
|
//
|
|
// arch_get_entropy exploits the uncertainty in the asynchronous boundary crossing
|
|
// between the CPU and a counter. To work correctly, the CPU and counter must be
|
|
// derived from clocks with relative jitter (PLL vs. OSC). Ideally the inner loop,
|
|
// up to 32 iterations, should consume a large part of the effective read period of
|
|
// the counter.
|
|
//
|
|
|
|
ARM_FUNCTION _arch_get_entropy
|
|
|
|
mov r1, r0
|
|
mov r0, #0
|
|
mov r2, #32
|
|
1:
|
|
mov r3, r2
|
|
2:
|
|
subs r3, r3, #1
|
|
bne 2b
|
|
ldr r3, [r1]
|
|
and r3, r3, #1
|
|
orr r0, r3, r0, lsl #1
|
|
subs r2, r2, #1
|
|
bne 1b
|
|
bx lr
|