Enthusiasts who started their hobby with the
Sinclair ZX81 (shame on you! Ed.) may remember the original
version of Great Britain Limited from Hessell Software. The player
assumes the role of Chancellor of the Exchequer and each year for
five years must carefully set the level of various taxes and social
benefits. At the end of five years a General Election is held where
only skilful Chancellors are re-elected to another five years in
office. Get re-elected often enough and you are in the running for a
Knighthood!
A permanent display keeps you in touch with the
economic state of the Nation, quoting such things as unemployment,
inflation, VAT, the balance of payments and, perhaps most important
of all(!), your popularity rating. In the second and subsequent
terms of office the going gets tougher and it takes considerable
skill to get your party re-elected as any Prime Minister will tell
you.
The Atari version is closely modelled on the ZX81
game with substantial additions to introduce colour and music. Two
black panels containing information and instructions are found at
the top and bottom of the screen, divided by a central panel of a
colour appropriate for your chosen Party. Who said you can only have
one background colour in Graphics 0?
A SPECIAL DISPLAY SCREEN
The program itself is quite straightforward but
some of you may be interested in details of the display list so that
you can use similar techniques in your own programs. I have included
three tables which give most of the details necessary. Table 1
contains the complete display list. The first three numbers, and the
number 16 which follows the two interrupts, represent blank scan
lines which produce the border around the display and are set to
divide the coloured panels of the display. Table 2 contains a full
list of numbers to include in display lists to obtain different
widths of blank border colour. Remember that in Mode 0, a normal
line of text is 8 scan lines in depth. If you wish to set screen
colours without having to type in the whole program, just type in
lines 760 - 880.
The Assembly Language program of the two display list
interrupts is
shown in Table 3, together with an explanation of purpose of each
machine code instruction. The machine code is stored in page 6, at
location 1536 onwards. The third number of the DLI corresponds to
the colour which is subsequently loaded into the background colour
register located at D018 Hex. The colour number is shown as a 0 in
Table 3 but before the DLI is used the colour DATA is POKEd into
the third byte of the DLI by program lines 910 - 930. The formula
for the colour data is : COLOUR NUMBER * 16 + LUMINANCE. For the
Conservative Party, for example, instead of using SETCOLOR 2,7,4 to
obtain blue, the machine code equivalent is 116 ('7 * 16 + 4). The
colour data is loaded into the hardware register for immediate
effect and not into the shadow register which is
only copied into the hardware register during the Vertical Blank
Interrupt occurring at the end of drawing the whole screen.
Since there are two DLI's during the drawing of the screen, each
loading the hardware register with a different colour, it is
necessary at the end of each interrupt to reset the start address of
the interrupt to be used next. The low byte of this address is
stored in location 512 ($200).
The most annoying thing about a program with multiple interrupts is
the presence of the cursor and the question mark on the screen. The
cursor is made invisible by POKE 752,1 and the question mark can be
avoided by opening a channel to the screen editor for input and
output at line 420. All inputs are then of the form INPUT #1,
variable name. The screen editor echoes the input to the screen so
that you can see what is typed in, and correct it if necessary, but
does not display a question mark.
IN CONCLUSION
The music is provided to offer light relief in
between your struggles with mounting inflation and unemployment.
When you have heard enough just press START.
The original algorithms for calculating the effects of taxes and
expenditures are unchanged from the ZX81 version. Now is the time
for all you budding economists out there to fathom out how the
algorithms work and to design some additional modules. How about the
selling of National assets or Nationalisation or the effects of
curbing local government spending?
ACKNOWLEDGEMENTS
My thanks to Simon Hessel for kind permission to
publish Great Britain Limited.
My thanks to Michael Doolan for help and advice with the music.
________________
|
|
|
 |
Table 1 - Display List - Mode 0 Screen
Each display list entry is numbered for the sake of clarity. These
line numbers are not entered into the program.
|
Line |
Display List Instruction |
Purpose |
| |
|
|
|
1 |
112 |
Eight blank scan lines |
|
2 |
112 |
Eight blank scan lines |
|
3 |
48 |
Four blank scan lines |
|
4 |
66 (64+2) |
Load Memory Scan plus one mode 0 line. |
|
5 |
64 |
Low byte of start of screen memory. |
|
6 |
156 |
High byte of start of screen memory. |
|
7 |
2 |
Mode 0 line |
|
8
- 11 |
2 (each line) |
Total 4 mode 0 lines |
|
12 |
130 (128+2) |
Display List Interrupt plus mode 0 line |
|
13 |
16 |
Two blank scan lines |
|
14 |
2 |
Mode 0 line |
|
15
- 25 |
2 (each line) |
Total 11 mode 0 lines |
|
26 |
130 (128+2) |
Display List Interrupt plus mode 0 line |
|
27 |
16 |
Two blank scan lines |
|
28 |
2 |
Mode 0 line |
|
29
- 31 |
2 (each line) |
Total 3 mode 0 lines |
|
32 |
65 |
Jump Vertical Blank (to following address) |
|
33 |
50 |
Low byte of start address of display list |
|
34 |
6 |
High byte of start of display list |
Table 2
- Blank Lines
|
Display List Instruction |
Number of blank
scan lines |
| |
|
|
0 |
1 |
|
16 |
2 |
|
32 |
3 |
|
48 |
4 |
|
64 |
5 |
|
80 |
6 |
|
96 |
7 |
|
112 |
8 |
Table 3
- DISPLAY LIST INTERRUPTS
|
Assembled code (Hexadecimal)
|
Assembly Language |
Purpose |
|
|
|
|
|
First Interrupt |
|
|
|
48 |
PHA |
Stores accumulator on stack. |
|
A9 00 |
LDA# 50 |
Load accumulator with colour
data. |
|
8D OA D4 |
STA WSYNC |
Synchronises interrupt with
start of a new scan line. |
|
8D 18 DO |
STA COLOR |
Stores colour data into hardware
register for background colour. |
|
A9 10 |
LDA# $10 |
Load accumulator with 16
decimal. |
|
SD 00 02 |
STA $200 |
Store accumulator in $200
(location 512) to direct the next DLI to the 2nd interrupt. |
|
68 |
PLA |
Restore the original contents of
the accumulator from the stack. |
|
40 |
RTI |
Return from Interrupt. |
|
|
|
|
|
Second Interrupt |
|
|
|
48 |
PHA |
|
|
A9 00 |
LDA # $0 |
Load colour number - 0 for
black. |
|
8D 0A D4 |
STA |
WSYNC |
|
8D 18 D0 |
STA |
COLOR |
|
8D 18 02 |
STA $200 |
Stores 0 in location 512 to
point to first interrupt |
|
68 |
PLA |
|
|
40 |
RTI |
|
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