Newsgroups: comp.arch
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: register save/restore
Message-ID: <1988Oct30.013510.16861@utzoo.uucp>
Organization: U of Toronto Zoology
References: <3300037@m.cs.uiuc.edu>
Date: Sun, 30 Oct 88 01:35:10 GMT

In article <3300037@m.cs.uiuc.edu> grunwald@m.cs.uiuc.edu writes:
>Many (most UNIX ) systems apply the convention that the callee must save
>any registers used in a procedure. Other systems dictate that the caller
>must save the registers. ... Is there an advantage?

As usual, it depends.  Ideally, one wants to save and restore registers
as little as possible, because it costs time and memory.  The caller
knows which registers don't have to be saved because they don't contain
anything interesting.  The callee knows which registers don't have to
be saved because he's going to leave them alone anyway.  Neither is
clearly superior for all situations.  It's not unheard-of to split the
available registers into a callee-saved group and a caller-saved group.
(What does MIPS do?)

The callee-saves bias in Unix is basically historical.  On the 11, there
were so few free registers that the calling sequence simply saved and
restored all of them, and doing this in the callee saved code space.
(This is a slight oversimplification.)  On the VAX, the wonderful all-
singing all-dancing standard calling sequence provided by the hardware
encouraged callee-saves.  Not everyone has bothered to rethink the issues
when changing processors.
-- 
The dream *IS* alive...         |    Henry Spencer at U of Toronto Zoology
but not at NASA.                |uunet!attcan!utzoo!henry henry@zoo.toronto.edu