11 Jul
2012
11 Jul
'12
8:10 a.m.
On 5 July 2012 19:53, Jecel Assumpcao Jr. <jecel at merlintec.com> wrote:
Alexey Toptygin wrote:
It's not really "the model presented by BASIC".
I mean, yes, there are user-defined graphics (UDG) characters, but
they weren't widely-used except in the very early days.
Spectrum BASIC has direct access to individual pixel-plotting (PLOT
command), line-drawing and arc-drawing (DRAW command), circle-drawing
(CIRCLE command), and setting foreground and background colours (INK,
PAPER, BRIGHT). You can also interrogate a point on the screen to find
its status (set/reset) or colour (POINT command, IIRC - I'm not sure).
Aside: This sort of functionality is why the 14-year-old version of me
found the Commodore machines crippled to the point of utter
un-usability and uselessness - none of this was possible in CBM BASIC.
It all had to be done with PEEK and POKE commands directly
manipulating memory locations. I'd already had a little experience of
that on the PET machines and I did *not* want to go back.
If you added Dr Andy Wright's Beta BASIC toolkit to your Spectrum,
then you got graphics "modes" - you could choose a 6- or 5-pixel-wide
font for 42 or 51 columns with a high degree of legibility, or 4-pixel
characters and 64 columns with poor legibility, or 3-pixel characters
without gaps between them for a fairly illegible 85 columns - but
which permitted porting BASIC apps from expensive American
"professional" machines with 80-column displays, or in
word-processors, something which could show a full-width A4 page's
worth of text. It also supported text windows with scrolling and other
luxuries, and could store arrays in the RAMdisk of the Spectrum 128,
meaning that your ~20KB basic program could manipulate ~80-90KB arrays
in RAM - very snazzy stuff for 1985 or so.
--
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The Spectrum isn't really bitmapped. IIRC you
define up to 20? 8x8 pixel
tiles (one bit per pixel) and can draw them in place of characters (i.e.
at any multiple-of-8 pixel vertical or horizontal offset from 0) with the
same color and bright/flash attributes as normal characters. That was the
case in BASIC, at least, maybe you could do more in assembly, but I don't
recall seeing a frame buffer in the memory map...
When the Spectrum came out, I had the impression that it was a text mode
machine with redefinable characters like many of its competitors. Given
that this was a popular hack to give the ZX81 "high resolution" graphics
and the way color is controlled in 8x8 pixel blocks, this was a
reasonable guess. But I learned otherwise a few years ago.
The only hardware mode is a simple 256x192 bitmap starting at 0x4000
which is modified by a 32x24 attribute map at 0x5800. The model
presented by Basic is what you described, but the model seen by assembly
language programs is very similar to what the Mac had.