Question

北极圈保护区boot计手册说,登记册封顶应当是零。为什么登记册的编号为零?

http://www.arm.linux.org.uk/developer/posting.php

CPU register settings
    r0 = 0.
    r1 = machine type number discovered in (3) above.
    r2 = physical address of tagged list in system RAM.

I browsed through the arch/arm/kernel/head.S but could not find the reason for that.

Answer 1

虽然我可以发现,在光栅栏邮寄名单或含水层来源中有任何提及证实了这一点,但我要猜测的是,这一价值正被用作《反导》版本,用于对《反导条约》的今后防伪。

ker子未来版本可能希望修改从boot子载荷中得出哪些论点:也许需要一个新论点来说明某些新的CPU特征,或现有论点之一需要稍微缩小。

这提出了一个严重的问题,即新车子是从旧车库中load起的:看守人如何知道正在通过哪些论点? 我们可以尝试并强制实施,新油轮只剩下新载荷,但这会在过渡期间造成几个头痛。 (波塔载荷由北纬度光线掩体小组以外的人员书写;还经常深入到硬件中,在某些情况下无法轻易升级)。)

更好的解决办法是保留登记册<代码>r0,作为ABI版本。我们现在坚持认为,<代码>r0永远是页: 1 以后的胎体可以检查<代码>r0,以确定其与另一个登记册中的价值如何解释。

Answer 2

Consistency and efficiency. Since setting a register to zero is a common operation, and ARM is typically used in constrained environments, there may be an improvement in code density. The instruction encoding for setting a register to an immediate value is longer than setting a register to the value of another register. Whether this makes much of a difference in practice is another question.

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