80-Bus News

As David Hunt has written a comparative review of the three colour boards for the 80-BUS, I have willingly left the detailed review of the Climax MV256 to him. Listing 1 is a program to run on it. This program draws a map of Ireland including the major rivers. Included are two extensions to this program. Listing 2, also written in MBASIC, is to be inserted into Listing 1 according to its line numbers. This draws the picture on an EPSON FX80 installed as the LST: device in Graphics Mode 4. This is slow, taking 27 minutes to draw the picture!

What happens in the printer driver is this: The Y coordinate is set to 255, and the X coord to 0. GETCOL(X,Y) is called to return the colour of the pixel at X,Y. If this is not black, then we calculate the value needed to display this bit if we print rows of 8 bits vertically. We work down 8 rows, in the first column, building up the value of the byte. Then we move to the next column, top row, and carry on. Having finished the screen-width, we transmit the graphics line selectors, and the entire row. Then on to the next group of 8 raster lines. Because of the looping involved, this is very slow! Listing 3 is an improved version of this. It patches into the MVLINK primitives and is written in assembly language. It should be inserted into MV2.MAC after INIT, and MVLINK.MAC reassembled and linked with M80/​L80. Using machine code, this performs the same task in 35 seconds! Note that there are two other entries needed to use this listing – an insertion of a call to DUMP in the jump table (p53, Appendix B, Microvector Manual), at address 02FA and the insertion of a new label in GETP(X,Y), page 59, label name GETCOL at address 049A (PUSH BC) in the MVLINK listing. This latter saves a call to GETPAR and READY, to speed things up slightly. It is important that the LST: device driver should not interfere with the data at all, otherwise spurious characters may be printed. This means that users of the SYS BIOS and the MAP80 BIOS will have to ensure that the page length is set to 0 by changing the byte in the BIOS at plpag:, see the system notes for details. If using the new primitive, I suggest that it live at MVBASE+99, and be called DUMP. It should be called with one parameter, the colour of the background of the screen, which will not be printed. All other colours will be. It is possible, using this primitive, to print an enlarged picture by drawing on screen four successive quadrants, printing each, and winding the printer back up after them using the reverse line feed. This is left as an exercise for you!

It is important when using the MV256 that the program be saved to disc before executing. If through some error or oversight one of the CALLs is not properly defined, it will be treated as CALL 0, and the program crashes, causing much weeping and gnashing of teeth. While there are methods of regaining the source code, it is easier to MVLINK, and LOAD "FILENAME"!

When purchasing the MV256, it is worth getting the disc with the driving primitives and demonstration program. The demo programs are quite impressive, and give one something to aim at.