tube (oversimplified). The beam is made to scan horizontally in sequen— tial lines across the screen and the whole screen is covered 50 times a
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and the by sma tlllte es tcznntpiliter 0 can ma ktherefore e. in between each horizontal scan of a line there is a small the delay horizontal blank. Also between each time the screen is drawn there is
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normal TV picture consists of 625 of the lines (in fact it consists of 312 interlacing, alternating lines). The computer display, to avoid overscanning the TV and losing data, consrsts of only 192 lines, Ieavmg a gap at top and bottom of the screen. Antic is able to control each scan line individually and up to 320 individual pixels horizontally. A pixel is a single point on the screen created A
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anbther delay the vertical blank. More of these later. The higher resolution modes (192 vertical resolution, say Graphics 8) use one scan line per horizontal row of the screen. However other modes use up to 16 scan lines per line of the graphics mode.
'Stored in
a
rather complicated way.
' .
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The scan lines used are:
mode
(1)
resolution
52
g 16
12
12 24 48 48 95 96 192 24
13 14 15
192 192
2 3
4 5 6 7
8-11
8
4 4 2 2 1
8 16
12
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—
you
mustsplititintotwoparts.Thisis
done by firstly finding the number of times 256 will divide into it and secondly the remainder. The first number is known as the high byte of the number and the
remainder is the low byte. They are stored in memory in the order low
byte, high byte. For example, for 42000 you get 42000/256=164 remainder 16. The high byte is 164 and the low byte 16. lf 42000 was the location of the display ?st then 560 would contain 16 and 561 would contain 164 (if there is no remainder then 0 must be stored in 560). Conversely, to find where'the display list is located you multiply the number in location 561 by “256 and add this to the number in location 560, that is PEEK(561)*256+ PEEK(560) gives the location of the display list.
Most display
short,
lists are very
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1
-
Decimal
Hex
112 112 112
70 70 70
next question how does The IS,Antic know what to display? The answer lies in the display list, a small machine code program interpreted by Antic to give the display. It tells the chip two main things: 0 The Antic graphics mode number for each line. ‘ memory location Of the screen .
h '5 "Orma'w and m?” created, ipulated by the computer 3 operating system and the Basic programmer can forget it. The whereabouts of the display list
)3lines )eachof8blank
-
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,
66
42
=64 (LMS Instruction)
+2(GraphicsOIine) 64 155
She isplay.
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a
1
.
.
modeline
,
stored in rather
complicated way, in memory locations decimal 560 and 561, because a computer does not work in decimal (base 10) as we do. It works in binary numbers (base 2). These are often expressed as hexadecimal (base 16) see Mike Bibby's Bit Wise article on Page 46 for an explanation of this. Every memory location in the computer can store a number between 0 and 255. Therefore to express numbers greater than 255 you must use two memory locations. So to store a number such as 42000 is
2
02
”
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65 32 156
_
Figure
40 )Screen memory location 9C )=54+156'256
I/.' Graphics
41
20 9C
)23linesthe same
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He. 23 Basic GraphicsOlines
=6.4+1Endofdisplaylist&JuMPto )
Memory location ofstart oflist
'
'
)=32+156*256
0 display list
,
July 1935 ATARI USER
25