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From: emjej@uokvax.UUCP
Newsgroups: net.micro.6809
Subject: direct page storage class
Message-ID: <3500134@uokvax.UUCP>
Date: Sat, 8-Mar-86 07:15:00 EST
Article-I.D.: uokvax.3500134
Posted: Sat Mar  8 07:15:00 1986
Date-Received: Wed, 12-Mar-86 02:14:03 EST
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Nf-ID: #N:uokvax.UUCP:3500134:000:2543
Nf-From: uokvax.UUCP!emjej    Mar  8 06:15:00 1986


[I'm posting this here because it's specifically 6809-related.]
[I hope that you will look at mod.os.os9 for generic OS-9 material.]

For people using the Microware OS-9/6809 C compiler, let me strongly
suggest you consider judicious use of the "direct" storage class.

Huh? you ask.  Well, I'm glad you asked that.  People familiar with
the 6809 will know what I'm talking about, but a short explanation--

minis and micros sometimes provide short addressing modes to refer
to a particular page, called the "direct page," which lets one get to
important data quickly.  Old-fashioned processors, e.g. the 4004 and
its many bastard offspring, and the PDP-8, made the direct page live
on page 0, so that, for a processor with a 64K address space, you'd
typically give the low-order byte of the address of a direct-page
datum, and the processor would effectively stuff 0 in the high-order
byte.

The 6809 is different.  It has a direct-page register that is used
for the high-order byte of direct page references, so that the
direct page may be any 256 bytes (aligned so that it starts at XX00
for some hex value of XX).  OS-9 gives each process its own direct
page (and the kernel has one, too); setting/saving the DP register
is, of course, just part of context switching.  The process's direct
page is the bottom page of its data memory.

The C compiler puts global variables at the bottom of data memory,
working up--so some indeterminate number of variables will show up
on the direct page in any case.  The problem is that they might not
be the ones that you, as the programmer, know or determine to be the
most important or most-used ones.

Microware's C compiler, therefore, allows one to specify that global
variables are "direct," i.e. to be given precedence for placement on
the direct page.  One can say, for example,

	direct char state;	or	direct extern char state;

Asking for too much stuff in the direct page works just like excess
register declarations, i.e. excess is ignored.

It *does* make a difference--I tried some judiciously-placed direct
declarations in a rewrite of the old CUG Unix ed-like editor, and
it shrank the code by about 500 bytes.  (I think the compiled code
came to 15.5 Kbytes at the time.)  Things got faster, too.

For portability, I'd recommend typing DIRECT and then conditionally
compiling with -dDIRECT=direct or -dDIRECT= as need be.  In a header
file, I use

#ifndef DIRECT
#ifdef OSK		/* 68000s don't have direct pages *
#define DIRECT direct
#else
#define DIRECT
#endif
#endif

					James Jones