PrintProg

Character Print





Paul Rhodes shows you how to print characters


with proportional spacing on your screen.





This program allows you to display text with proportional


spacing of characters on the screen or ZX Printer. It is


called using the Print, LPrint, List and LList commands, as


for normal printing.


The characters are printed with differing widths, depen-


ding on their shape. For example, ! is narrower than a W.


The routine can be adapted to give a set number of charac-


ters per line - for example 40, 64 etc - by setting all


characters to be the same width.


The program is operated via the Print #9 command - simi-


larly for LPrint #9, List #9 etc. The only restriction is


that, especially if Interface 1 is connected, functions


such as CHR$ and STR$ may cause the computer to crash. This


appears to be due to the ROM, as the crash occurs /before/


the print routine is called. To avoid this problem, use a


string as a "buffer". For example:


LET a$=CHR$ n: PRINT #9;a$


instead of:


PRINT #9;CHR$ n


[I haven't been able to replicate this bug, but that may


mean only that it is present only under some circumstances.


There is certainly a known bug in the STR$ ROM routine,


although I have never heard of a similar one for CHR$.]


[There were instructions here for entering the code and


proportional character set. There are different versions


for the 48K and the 16K machines. These are on the TZX that


goes with this text file, as "PrintCde48"CODE 63744,1536


and "PrintCde16"CODE 30976,1536 respectively. Don't forget


to CLEAR <address-1> before loading!]





Setting up channels





There is a different method for setting up the channels


if you have an Interface 1 connected, so if you have, skip


the next paragraph and enter listing 2. For those of you


without the interface:


RANDOMIZE USR 63744 (or 30976)


will set up the #9 channel.


If you have an Interface 1 connected, start reading again


here! Due to the way the Interface 1 affects the error


routine, the above method is very risky. You can use it,


but if you do the system is liable to hang up whenever


there is an error, or when a Microdrive command is used -


CLS# and CLEAR# are OK, though. So far as I know, this


problem is unavoidable. An alternative method must there-


fore be used. Listing 2 [on the TZX as "if1 init"] is a


subroutine which will set stream 9 to point to the Print


Prog channel data.


However, the data is not stored in the channel area, so


the routine must be called again every time the Micro-


drives, network etc. have been used. Also, if the shadow


systems variables have not been inserted when the routine


is first called, it must be called again after they have -


that is, after any error, or any Interface 1 command.


You should now be able to print to the screen in the new


layout using Print #9. Apart from the problem with CHR$


etc. mentioned above, most of the normal control characters


can be used. The routine operates with colour controls,


Inverse, and position controllers. The number in TAB state-


ments - and the second number in an AT statement - has a


range of 0-247 - 248-255 will continue on to the next line.





Text at pixel positions





This means that text can be printed at pixel positions


across the screen. Positions down the screen have the


normal 0-21 range. CHR$ 6 - the "Print comma" - puts the


cursor at the beginning of the next print zone - either


half way across the screen, or at the beginning of the next


line - as normal, and CHR$ 13 will move the print position


to the beginning of the next line.


When used with the printer, the buffer is copied whenever


a new line is started. As you can see, there are no major


differences between using this routine and using the normal


Print command.


The character set is stored as it is in the ROM, except


that the first byte of data for each character stores the


width. The remaining 7 bytes store the bit-pattern, which


starts one bit from the left of the character - see dia-


gram 1. The top line of the character is therefore always


printed as a 0 (blank).





Diagram 1


0 0 0 0 0 1 0 1 =5 <1st byte


0 1 1 1 0 0 0 0 =112 <2nd byte


0 1 0 0 1 0 0 0 =72 <3rd byte


0 1 0 0 1 0 0 0 =72 <4th byte


0 1 1 1 0 0 0 0 =112 <5th byte


0 1 0 0 0 0 0 0 =64 <6th byte


0 1 0 0 0 0 0 0 =64 <7th byte


0 0 0 0 0 0 0 0 =0 <8th byte


a b b b b c c c


The first byte determines how many columns (from the left)


will be printed.


a=one column blank for spacing between characters


b=These columns make up the character


c=These columns are ignored.





Poke the first byte





To set 40-column printing, Poke the firstbyte of each


character with 6 - listing 3 [on the TZX as "40 columns"].


Then Poke the "right-hand margin" addresses - are marked on


the hex listings - with 239 - this is 40*6-1. [I can't give


the entire hex listings nor underline any numbers in this


text file, but as a note underneath listing 3 made clear


anyway, there are two of these addresses; for the 16K, they


are 31367 and 31638; for the 48K, POKE 64135 and 64406. In


any case, the program itself will POKE those addresses for


you.] You will probably also want to redefine the character


set, as the specially designed "proportional spacing"


character set can look very odd when printed with regular


spacing.


Other column widths can be set similarly.