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From: emigh@ecsvax.UUCP
Newsgroups: sci.bio
Subject: Re: information content of DNA
Message-ID: <2859@ecsvax.UUCP>
Date: Fri, 3-Apr-87 22:21:52 EST
Article-I.D.: ecsvax.2859
Posted: Fri Apr  3 22:21:52 1987
Date-Received: Sun, 5-Apr-87 08:57:42 EST
References: <2840@ecsvax.UUCP> <11189@teknowledge-vaxc.ARPA> <978@aecom.UUCP> <2844@ecsvax.UUCP> <7659@ut-sally.UUCP> <425@haddock.UUCP>
Reply-To: emigh@ecsvax.UUCP (Ted Emigh)
Organization: NC State University
Lines: 29

In article <425@haddock.UUCP> johnc@haddock.ISC.COM.UUCP (John Chambers) writes:
>
>I wonder if anyone has ever built a computer with the possibility of multiple
>"reading frames".  Consider an 8-bit memory, but a 16-bit instruction size.
>If you start executing at address A and at A+1, you get two possibly very
>different programs.  Can any real-life processors do this?  It happens
>with DNA quite often.

When I was still using Microsoft CP/M FORTRAN, I ran across some of their
assembler programming in the FORTRAN libraries that did this.  The 16 bit
load was 1 byte for the instruction and 2 bytes of data.  The 8 bit load
was 1 byte for the instruction and 1 byte of data.  A segment of code
might look like:

Location      1     2     3
If we started executing at 1, it read as
Instructions LD HL Low   High   (Low and High are the 2 bytes of data)
If we started executing at 2, it was
Instructions  xx   LD A  Data

The HL value was always discarded (a slow NOP), and this presumably saved
then a jump at some point.  Needless to say, it was a mess to read and
very poor programming practice.

-- 
Ted H. Emigh     Genetics and Statistics, North Carolina State U, Raleigh  NC
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