VDU 19

VDU 19





Andrew Daines,


Bungay,


Suffolk.





These machine-coded routines are designed to emulate the


VDU 19 command found on BBC and Electron computers. The


first routine performs a universal ink change without dis-


turbing the text already on the screen. It will not search


for and replace any colour but it will change all the text


to blue, for example.


The second routine is similar to the first but changes


the paper colour instead of the ink, again this routine


leaves any text unchanged. After this comes the third


routine, this is more complex than the previous two. It


searches through the text and if it finds a certain colour


it will change it to another, again this leaves all text


unchanged.


Finally comes the fourth routine which searches through


paper colours and if it finds a specific colour it changes


it.


Now you know exactly what each routine does you must type


them in. First type in and run the program provided. This


will ask you for the address you want the machine code to


start from, on a 16K machine 30000 is a good origin and


60000 on a 48K machine. [In fact, there are several hard-


coded addresses inside the code, so as published it won't


work anywhere else than at 60000.] When this address has


been entered ten more addresses will be displayed, the


meanings of which are given here:


Address 1: Poke this with the new ink colour.


Address 2: RANDOMIZE USE for routine one.


Address 3: Poke this with the new paper colour.


Address 4: RANDOMIZE USE for routine two.


Address 5: Ink colour to search for.


Address 6: New ink colour.


Address 7: RANDOMIZE USE for routine three.


Address 8: Paper colour to search for.


Address 9: New paper colour.


Address 10: RANDOMIZE USE for routine four.


Addresses 1 and 2 refer to routine one.


Addresses 3 and 4 refer to routine two.


Addresses 5, 6 and 7 refer to routine three.


Addresses 8, 9 and 10 refer to routine four.


After printing these addresses the program will proceed to


poke the machine code and after a few seconds you will


either see the message "code O.K...." in which case you can


save and verify the code, or the message "Checksum error;


check data" which means that you have mis-typed one or more


of the numbers and the program will stop allowing you to


check and rectify the data. When you are satisfied the data


is correct, re-run the program.


The basic programme will then re-new itself [except that


the one on the TZX won't pull that cheap trick], you can


then type:


CLEAR st-1


where st is the origin. And then type LOAD ""CODE and load


back the machine code in memory, then you are free to use


the routines.