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Message: 5490

Author: andrewbroad

Date: 12/02/2006

Subject: Re: How best to represent hexadecimal? #

 

After sleeping on the problem and meditating further, I've decided
to use # as a prefix for hexadecimal literals.

If I see &86B0, $86B0 or #86B0, my brain is much quicker to tell me
that #86B0 contains 4 rather than 5 digits.

# is not the most common hex-prefix, but it's the one Geoff Eddy
uses in his disassemblies and on his website, and it's also used in
HTML as a prefix for hexadecimal colour-attributes.

I mentioned that some assemblers use $ to mean the current value of
the location-counter. Another (not Z80) assembler I've seen uses #
to denote an immediate operand - in denary - but I'll just have to
live with that.

I often use #n to mean "number n" (in denary), but I've never done
that in a MM/JSW context (I used this notation in my PhD thesis, and
I continue to do so when discussing tennis-rankings).

john_elliott_uk wrote:

(a) &DE. I like the idea of having a one-character non-
alphanumeric symbol as a prefix, as long as there's no ambiguity
with the use of & as the bitwise-AND operator. Unfortunately if
you type &DE on a command-line, it strips off the & unless you
type "&DE".

Depends on the command line. In UNIX shells, it runs the task in
the background.

And in Windows XP's Command Prompt, it's used to separate multiple
commands on one command-line. I actually managed to start
RealSpectrum by using & in the vicinity of FILENAME.TAP!

# doesn't have such a drawback (at least not in Windows XP's Command
Prompt).

(e) DEh (as seen in assemblers, along with 222t for denary,
11011110y for binary and 336o for octal). But this is long-
winded, and again could be misinterpreted as a variable-name
unless written as 0DEh - yuck!

No Z80 assembler I've used allows hex numbers in this form to start
with a letter, so if you want something that actually assembles
you'd have to include the leading zero.

I've revised SPECSAISIE Disassemble to use the # prefix for all
hexadecimal numbers > 9 when the command-line flag -h (output in
hexadecimal) is given.

I've written a new method ByteSupport.parseInt that acts as a
wrapper for Integer.parseInt, but recognises &, $, #, 0x and 0X as
hexadecimal prefixes, and h and H as hexadecimal suffixes. So far
only SPECSAISIE Disassemble calls this method, but soon SPECSAISIE
will use it for all numeric command-line arguments.

SPECSAISIE Assemble - when I write it - should recognise all these
hex-notations as input (but will no doubt require leading zeroes for
hex-literals starting with A-F when using the h suffix).

-----------------
128K memory-banks
-----------------

My hex-notation will also have to interact with my bank-notation
(when I implement SPECSAISIE's model of 128K memory).

e.g. John Elliott wrote "C300h in bank 3"

3:C300h
&3:C300 }
3:&C300 } ditto re. $ and #
Ox3:C300
3:0xC300

Since the bank is always in the range 0-7 and therefore looks the
same in denary, hex and octal, putting the indicator on the side
with the bank number rather than the address might cause
confusion.

I guess I'll put # on the right-hand side then:
3:#C300 (accepted as input and generated as output)
#3:C300 (rejected because C300 doesn't parse as an integer)
#3:#C300 (accepted as input, but the first # is unnecessary)

So SPECSAISIE will have a method parseAddress which will call
ByteSupport.parseInt for the substrings to the left and right of :
(and will also accept 48K memory-addresses, e.g. #C300 on its own).

And if you used a convention that addresses formed b:xxxx were
hexadecimal, and addresses formed b:nnnnn were hex, you wouldn't
need any other indicator.

But that is inelegant and Spectrum-specific, and would break down if
I ever did want to address ROM (addresses < 16384 = #4000) - to
access the standard font, for example, or to disassemble ROM.

Hex 5:86B0: C3 CA 87
(5: is unnecessary in this example and incorrect; 86B0h is in bank 2

D'oh! :-o

--
Dr. Andrew Broad
http://geocities.com/andrewbroad/
http://geocities.com/andrewbroad/spectrum/
http://geocities.com/andrewbroad/spectrum/willy/

 

 

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