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By David E Mentley

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Bytes 4 and 5 are the initial address of the program to be loaded. This can be anywhere except where a conflict with the Operating System may occur. Byte 6 is a machine code to be executed after the boot is loaded. The OS contains 40 BOOT-UP PROCEDURE the program to read these bytes so this instruction comes from the OS ROM. Since byte 1, the number of sectors to read, is limited to the range 0 to 255, only 255 X 128 bytes (32K) can be booted from this boot program. This means that an additional program is needed to load in longer code.

50 CHARACTER SETS Memory location 756 Contents = EO-----, EOOO 00 Points to E001 06 '---~~ Each character is comprised of 64 bits of information which tell every one of the pixels in an 8 x 8 block whether to be on or off. The pointer in 756 points to the first byte ofthe character set data and every eighth byte is the start of a new character. You can change the memory to which the pointer points by POKEing location 756 with another number. Try the following experiment in BASIC. 0. This will make the data in page 0 become the character set data.

This common error is very difficult to track down. To see what the uppercase letter set looks like, RUN the following program: 10 FOR X=65 TO 90: PRINT CHR$(XJ : NEXT X Type in the program and then type RUN . CHR$(125) - The "CLEAR SCREEN" symbol is an inverse upward-left curving arrow in double quotes and it is often printed out as a bracket on many printers. A clearer way of including a CLEAR SCREEN command in your BASIC program is to substitute: PRINT CHR$(125J; for the Escape Shift-Clear key sequence.

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