Characters

Most displays are 64 characters wide.


Jon Ellis squeezes more into the Spectrum screen.





ZX Condenser





One of the many annoying features of the Spectrum is the


screen layout, 24 lines each of 32 characters, of which


only 22 lines are usually available to the user. Many


computers have the facility to put the more standard 40


characters - (or even 80 characters) - on a line. This


utility alters the pitch of the Spectrum printing to give


nine different character per line ratios. The routine also


enables you to position text on the screen and to give it


colour.


Although non-standard, 32 characters per line is a


logical development from the way in which Sinclair designed


the Spectrum screen. As you will know from user-definable


graphics, a character on the Spectrum can be represented by


a eight by eight grid of pixels. Since there are 256 pixels


in the x direction, we may fit 256 divided by eight, or 32,


of those pixel grids across the screen. What this utility


does is to alter the size of the pixel grid used to repre-


sent a character.


If we decide that a character is to be made up of a six


by eight pixel grid, then we will be able to fit 256 divi-


ded by six, or 42, characters per line, taking the integer


part of the division only. Table 1 shows the effect of let-


ting the size of the pixel grid from six by eight to 15 by


eight. Note that we are only altering the size of the grid


which defines a character, not the size of the shape with-


in the grid - i.e. what is being printed.





Table 1.





Size of grid Characters Value of


per line F


6 x 8 42 6


7 x 8 36 7


8 x 8 32 8


9 x 8 28 9


10 x 8 25 10


11 x 8 23 11


12 x 8 21 12


13 x 8 19 13


14 x 8 18 14


15 x 8 17 15





It is easy to print characters in grids larger than


eight by eight - all that needs to be done is to print each


character of the text as normal, but inserting a gap of


several pixels between successive characters. However, it


is slightly more difficult to condense print from the stan-


dard print. How do we make the grid smaller? The answer to


that can be found by looking at the design of the standard


Sinclair character set.


Taking a non-graphic character such as the letter 'm',


as in Figure 1, we see it has a border of blank pixels com-


pletely around the letter shape. What the program does is


to clip off the left and right hand edges for the six by


eight grid, or just the right hand edge for the seven by


eight grid. The routine prints the first character of the


text, and then moves along six, or seven, pixels before


printing the second character; that procedure is followed


for all the other characters in the text string.





+---+---+---+---+---+---+---+---+


| | | | | | | | |


+---+---+---+---+---+---+---+---+


| |###|###| |###| | | |


+---+---+---+---+---+---+---+---+


| |###| |###| |###| | |


+---+---+---+---+---+---+---+---+


| |###| |###| |###| | |


+---+---+---+---+---+---+---+---+


| |###| |###| |###| | |


+---+---+---+---+---+---+---+---+


| |###| |###| |###| | |


+---+---+---+---+---+---+---+---+


| |###| |###| |###| | |


+---+---+---+---+---+---+---+---+


| | | | | | | | |


+---+---+---+---+---+---+---+---+


Figure 1.





[This was followed by an explanation of how to enter and


save the code, including a short demo program. I've omitted


this, as it is all readily entered on the TZX. The Demo


program works on both 48K and 16K and loads the correct


CODE file. The code readers for both sets of code - identi-


cal except for quite a few hard-coded addresses, so they're


not relocatable - are on the TZX, as well.]


A brief explanation of the assembly language listing,


stage by stage follows. [The numbers are the addresses in


the 48K code. These were absent in the magazine. I haven't


copied the listing, as most good emulators will provide a


disassembly, but that does mean that the labels are not


much use as reference. Hence the addresses. For the 16K


addresses, subtract 32730.]


FINDX: Routine which uses SCAN to find the value of the


64810 variable x, providing that it can be found in


small integer form.


FINDY: Finds the value of small integer y.


64828


FINDF: Finds the value of small integer f, giving the


64842 report 'Q Parameter error' if the value is outside


the range 6 to 15.


FINDS: Finds the start address and length of the text


64866 string s$.


SCAN: Subroutine which searches the variable area for the


64880 variable specified in VARSC. If the variable speci-


fied is not found then the report '2 Variable not


found' is given. Note that the routine distinguishes


between, for example, x as the control variable of a


FOR-NEXT loop, and x as a simple numeric variable.


INIT: This calculates the maximum number of pixel grids on


64954 a line (the number of pixels in the x direction is


defined by the simple numeric variable f).


START: This takes the characters from the string s$ sequen-


64985 tially and prints them, such that the specified num-


ber of pixels occur between successive characters.


If there are more characters in the text string than


can fit on the line, then the routine will automati-


cally print to the end of that line and then carry


on printing on a line eight pixels down. The routine


ensures that a character may never straddle a line.


If the message was begun on line 0 (the bottom of


the screen) and has to be continued on the next


line, or indeed, if any message runs out of space


on the screen, the routine will automatically scroll


the screen up eight pixel lines with no 'scroll?'


prompt.


CHRS: The first part of this subroutine finds out which


65075 kind of character is to be printed: control charac-


ters, those with codes less than 32, are printed as


normal; keyword tokens, characters with codes great-


er than 164, are not printed at all; any remaining


characters are to be printed using the routine.


Before a character can be printed the routine must


find out where the data defining the character is


stored. If the character is part of the 'normal'


character set, i.e. its code is between 32 and 127


inclusive, then the bytes defining it are in the


area starting at the address given by the system


variable CHARS (23606)+256. If the character is a


user-definable character, then the defining bytes


are in the area pointed to by the system variable


UDG (23675). If the character is a block graphic


then the defining bytes are created by manipulating


its character code. In all cases the 8 defining


bytes are transferred to the workspace.


WHERE: This uses the current row and column values to find


65184 the correct address in the display file for each of


the eight bytes of the character. It also colours


the appropriate attribute squares.


The position at which text is started is defined by the


values of the simple numeric variables x and y. X is the


x coordinate of the pixel at the bottom left of the first


character, and y is the y coordinate. If y is greater than


167 then part or all of the start of the text will be


printed off the top of the screen.


The simple variable f contains the number of x pixels


per character grid. For the relation between f and the


characters printed per line, see Table 1.


The variables x and y follow the coordinate system for


high resolution plotting as opposed to the PRINT AT coordi-


nate system. That gives a range of 0 to 255 for x, and to


175 for y.


S$ contains the text to be printed and must be a simple


string. To print an element of a string array, for example,


element three of array x$, use LET s$=x$(3), and then


proceed.


To set the colours of the text a PRINT line may be in-


serted before the call to Multipitch:


PRINT PAPER 3; INK 7; FLASH 1; RANDOMIZE USR 64810/32080


would print the text in flashing white on magenta. If the


required attribute value is known then it may be POKEd


directly into ATTR T (23695).


Note that unless specified then the value will default to


that held in BORDCR (23624).


It is a good idea to ensure that the parameters x, y


and f are being held in small integer form, especially if


their values are derived by calculation, by inserting the


following line:


LET x=INT x: LET y=INT y: LET f=INT f


The colours of the text are always loaded into the


attribute file in such a way as to ensure that all charac-


ters in the text are affected.