Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!pyramid!hplabs!otter!kers
From: kers@otter.hple.hp.com (Christopher Dollin)
Newsgroups: comp.arch
Subject: Re: Auto-shifted registers
Message-ID: <780006@otter.hple.hp.com>
Date: 23 Feb 88 07:53:53 GMT
References: <1370@vaxb.calgary.UUCP>
Organization: Hewlett-Packard Laboratories, Bristol, UK.
Lines: 34

"radford@calgary.UUCP (Radford Neal)" says

|Several instruction sets, such as the 68020 and the VAX, have
|an "indexed" addressing mode, in which a register is shifted
|left by 0, 1, 2, or 3 bits before being added to a displacement,
|allowing easy access to arrays of ints, etc.
|
|I has occurred to me that this "auto-shifting" might be useful
|for ALL register references. Every reference to a register as
|a source operand would be accompanied by two bits giving a
|shift amount - equivalet to a multiplication by 1, 2, 4, or 8.

The Acorn Risc Machine has some of this.

The data manipulation instructions have the general form

    Target := Source1 Op Source2

where Target and Source1 are registers, and Source2 can be an immediate value
(an 8-bit field on an even bit boundary) or a register shifted left, right 
(logical or arithmetic), or rotated right, either by a small constant (0..31)
or by the content of another register (this adds an extra cycle).

The simple load/store instructions compute the memory address as

    Base {+|-} {12Bit | Register_with_Shift}

12Bit is a 12bit byte offset, RegisterWithShift is a register possibly shifted
(left/right (log/arith), rotated right) by a constant 0..31 (the register case
was eliminated from the second-generation chip as no-one ever used it and they
saved some chip area).


Regards,
Kers                                    | "Why Lisp if you can talk Poperly?"