The UDG Bank

The UDG Bank





84 UDGs must be enough for anyone.


B.J. Kamphuis shows us how to achieve


this on a Spectrum.





User Defined Graphics are a valuable and relatively easy


means of producing arcade action, background pictures or


business graphic details. Although twenty one are easily


obtainable, it is surprising how often you find you could


do with a few more.


There are many ways of getting an increased number of


UDGs and B.J. has adopted the method of storing four banks


which are transferred to and from the set UDG area of


memory as required. Now if 168 bytes have to be moved by


BASIC a noticable delay is introduced so a short machine


code routine is used to keep the program moving smoothly.


The UDGs are stored in four banks named, not surprisingly,


bank1, bank2, bank3 and bank4. Every bank can be trans-


ferred to the 'active' or normal UDG set and back.


To define a bank all you need to do is set them up as


ordinary UDGs from BASIC as normal, then use this program


to store the characters in the memory reserved for that


bank. To move a bank of UDGs to the storage area all you


have to do is use the command, either from a BASIC program


or as a direct command: RANDOMIZE USR address: REM W1


Now 'address' is either 64000 or 31232 depending on


whether you have a 16K or 48K Spectrum, and the 'W' after


the REM tells the machine code routine to write from the


normal UDG area to store, and the '1' tells it which bank


to use. Therefore when setting up your UDG banks you will


use W1, W2, W3 and W4 depending on which bank you are


working on.


The opposite effect, that of loading the normal UDG


area with the characters in a bank, is achieved by using


RANDOMIZE USR address: REM L1 - note 'L' instead of 'W'.


Once the banks have been set up then save the whole four


sets plus the machine code with the command SAVE "udgs"CODE


address, 1535 and to use in a program include after the


program has loaded the commands CLEAR address - 1:


LOAD ""CODE.





[ The TZX contains, for each memory size, the program from


the article which loads the code, followed by the machine


code plus four sample UDG banks as a CODE file. (The UDGs


are identical for both versions. Providing eight different


sets would be a bit much even for me.)


Mind you, why you would go to all this trouble instead of


simply poking the System Var UDG (23675/6), I don't know. ]