(C) Ivan Roshchin, Moscow, 1998
Features video controller ZX SPECTRUM
What will be seen on the screen, if we write to the
addresses # 4000 - # 57FF any information, and attributes
reset? And if zero # 4000 - # 57FF and install PAPER equal to
0, and INK how you like? Common sense suggests that in fact,
and in another If we do not see anything at all. All
this is so, but the following program
proves the opposite.
LD HL, # 5800
LD (HL), L
LD DE, # 5801
LD BC, # 2FF
LDIR
LD HL, 0
LD DE, # 4000
LD BC, # 1800
LDIR
LD BC, # 7FFD
LD A, # 17
OUT (C), A
LD HL, # C000
LD (HL), L
LD DE, # C001
LD BC, # 17FF
LDIR
LD HL, # D800
LD (HL), 7
LD DE, # D801
LD BC, # 2FF
LDIR
LD BC, # 7FFD
M1 LD A, # 18
OUT (C), A
LD A, # 10
OUT (C), A
XOR A
IN A, (254)
CPL
AND 31
JR Z, M1
RET
This program fills in the 1 st screen contents ROM and
resets the attributes (such way, the screen can not see
anything), then clears the 2 nd screen and sets He attributes
July INK, PAPER 0 (on 2-m screen can not see anything). The
program then begins to rapidly switch between the 1 st and 2 nd
screens, and here we see the following picture:
- - -
- - -
- - -
- - -
- -
- -
- - -
Apparently, the following occurs: at
displaying an image on screen bytes from a screen memory,
together with his attribute falls into a special shift register
but it may happen that the video controller will take a byte
from the screen memory 1 screen, and then the program will
switch the screen and the attribute byte is taken from the
already 2 nd screen (or vice versa). In both cases, the screen
shows white streaks.
I wonder whether this effect is observed
on other computers, but "Pentagona128? And the emulators? Can
we find for it any use?
Now we'll show you how I discovered this
effect. It was like this: I wrote a program
that show some picture on the
1-m screen, and then displays the following
Zoom in on the 2-m screen, and then again at
1-m, etc. Here's how this program works
(Active screen isolated double frame):
1) A 1-m screen, some picture on the
2-m screen with another picture, hidden under
attributes INK 0, PAPER 0.
2) Clear 1-th screen and set
7 INK, PAPER 0.
3) Turn on the 2 nd screen, and on it at
moment, a horizontal white
line length of about 8 pixels.
-
4) After HALT 'and for the pictures on the 2-m
screen, set its attributes.
*
