Your Computer type-ins

[Mike]

DEFENDER

Data entry and debugging by Mike

Compute mit, February 1986.

This is a type-in game for the unexpanded VIC-20 from a German computer magazine, which had been published as an hex-dump spanning four pages. In those days, I'd never have had the patience to actually type it in by hand, but nowadays, in the Ages of OCR, I thought it might be worth a try. The listing contained checksum bytes, which hopefully (and indeed!) helped to catch errors - to a degree. However, the OCR *had* difficulties to discern '8', and 'B', and the checksum algorithm would not notice two simultaneous exchanges of these in one line.

When I had finally found all these errors, the game still didn't work: as it appeared, the first 14 bytes had been corrupted by something that looked like two BASIC variable entries. The remainder of the following code resembled a joystick routine, and from this I tried to guess, what was missing. Indeed the replacement routine I thought out is *exactly* 14 bytes in length, and quite possibly truly reconstructs the original contents.

Here's the corrected hexdump. Download DEFENDER as one of the 21 games in my unexpanded type-in collection.

The game is controlled by joystick. Press fire in the title screen to start the game. When the game is over, press left shift to restart.

The translation of the game's instructions is given below.

Cheers,

Michael

Alien invaders have landed in the Sahara desert. To reach the most important capital cities of Earth, they drilled a long tunnel under the Atlantic to America. By chance the tunnel exit has been found, and you've been assigned the task to destroy the alien invaders in the tunnel.

But beware! The enemies shoot at you, and the tunnel is secured by mines and barriers. Aim to get as many enemies you can, before they get you.


DEFENDER for unexpanded VIC-20


The program is entirely written in machine language, and uses 3 K of the available memory. For this reason, it is impossible to type in the program as DATA lines.

To type in the program nevertheless, I wrote a small monitor, which only needs 384 bytes, and so fits into memory besides the program.

The monitor is put into DATA lines. To get it into memory, proceed as follows:

Code: Select all

NEW
POKE44,18:POKE4608,0:NEW

Type in, and start the DATA program.

Code: Select all

POKE44,16:NEW
10 SYS4324
POKE46,18:POKE45,0:CLR

Saving the monitor:

With M aaaa the memory contents at aaaa are displayed, with M aaaa bbbb the range between aaaa, and bbbb is displayed. Each line contains four data bytes, and one checksum byte. If the data bytes, and checksum byte don't match, the line is rejected.

When the program has been typed in completely, the monitor is left with STOP/RESTORE.

Then enter 'NEW', and '10 SYS 6656', and also 'POKE 46,29:POKE 45,240:CLR', and save
the program.

The program has been stored on tape. The monitor, and the DATA loader of the monitor have also been saved.

To print the HEXDUMP:

Load DEFENDER, load the monitor, enter 'OPEN 4,4:CMD 4', 'RUN', and 'M 1200 1DFF'.

[regor97]

GRIDMANIA
by Damian Mc Comb

Listing kindly supplied by Kweepa
Data entry and debugging by regor97

Your Computer, October 1983

Screenshot


Gridmania is an Amidar-type game that runs on an unexpanded VIC-20. Other than a BASIC loader the game is written in machine language. Unfortunately some of the characters in the listing are barely legible. After some debugging the game seems to run OK. It will eventually freeze if left on the GAME OVER screen, but this appears not to affect actual game play. GRidMAnia.zip contains:

GRIDMANIA.prg - BASIC loader (program one)
GM.prg - ML program

Note: in BASIC, to save the ML part to tape you need to enter POKE43,1:POKE44,16:POKE45,0:POKE46,30:SAVE"",1,1

Gridmania.tap - emulator ready tape version

all the above files are based on the original listing


Gridmania.d64 - modified loader for disk version


The following instructions are extracted from the magazine article and include how to adjust game speed.

------------------------------------------

To control your grid-glider you can use a joystick or a large selection of keys.

(not mentioned in the original magazine article is that to start play from the "GAME OVER" screen you can press one of the following keys: F1, F3, F5, F6, RETURN, CRSR down or CRSR right.)

The key at the top left of the keyboard freezes the game indefinitely until pressed again.

There are eight different kinds of alien attackers and they start off fairly slowly at level 1 and get faster with each level. Each level has two phases. Phase one has six of the aliens after you and phase two has eight of them. After that it goes back to six, but the level goes up one and a new kind of alien appears.

Your task is to completely cover the white grid with black and then return to home base in the quickest time. If you return to base before completing the grid you will lose a life. You cannot destroy the pursuing aliens so you must avoid them.

If that wasn't enough against you there is also a continually decreasing energy supply at the bottom of the screen. If this falls empty you lose a life. However when you complete a grid and return to base you will get a bonus for remaining energy. You will have to be quick though since this is the only way you can score.

The lowest high-score the game will register is 1500 and you will also gain a free life at this score.

Incidentally you can speed up or slow down the game with the following two locations:

6131 = speed of your grid-glider.
14 is normal, any less is faster.
6057 = speed of aliens.
14 is normal.

In order to change these you will have to delete the part of program one which disables the run stop/restore, i.e. the last five items of data in line 200 - A9,02,8D,1E,91.

Whenever you do break out of the game, to resume play SYS 5915.

[Jeff-20]

South Africa?! No way! Welcome!

[ravenxau]

These look great! I find type in programs so much more interesting than carts for some reason!

Maybe someone (maybe a mod) should separate program postings and discussion like the Italian type-in threads.

Type-ins are great - They were a great way to distribute programs back in the day. I can't begin to remember how many programs/games I collected from just photocopying pages from magazines in the school library. Also when you played a game and thought "Wow! I wonder how the programmer did that?", all you had to do was sit down with the listing and work it out. I learned 90% of what i know from programming during these formative years.