ZX Characters

ZX Characters





Trevor Terrell and Robert Simpson provide


character forming advice to help Spectrum


users define their own graphics.





A display of the character set of the ZX Spectrum obtained


using the three lines of Basic


10 FOR x=255 TO 32 STEP -1


15 PRINT CHR$ x;


25 NEXT x


is shown in figure 1. From the character set summary in the


Sinclair manual it may be noted that each character is


assigned a unique code in the range 0 to 255. However some


codes are not used, and others are used as control charac-


ters. There are only 224 characters that can be displayed


on the screen, - codes 32 to 255 inclusive - see range of x


values in line 10 of the program.


The characters having codes in the range 32 to 127 inclu-


sive, are stored in a block of read only memory - ROM -


between memory addresses 15616 to 16383 inclusive. Each


character is defined by the contents of eight successive


memory bytes.


The space character - code 32 - is stored in the eight


bytes in the address range 15616 to 15623, and successive


codes are correspondingly stored in successive blocks of


eight bytes. The eight bytes of any of these characters are


stored in memory ranging from address 15616 + (8*(code-32))


to address 15623 + (8*(code-32)). For example, the + cha-


racter - code 43 - is stored in the memory address range


15704 to 15711. You may Peek into these memory locations


using





10 FOR x=0 TO 7


15 PRINT PEEK (15704+x)


20 NEXT x


25 STOP





to obtain a display of the contents of each memory loca-


tion. The values are


Memory Displayed Memory


address contents contents


decimal binary


15704 0 00000000


15705 0 00000000


15706 8 00001000


15707 8 00001000


15708 62 00111110


15709 8 00001000


15710 8 00001000


15711 0 00000000


When the ZX Spectrum displays this + character it inter-


prets the bit pattern such that 0 is paper and 1 is ink,


and it is displayed using an eight-by-eight picture-element


- pixel - grid, as shown in figure 2.


Program 1 listed below may be used to display, for an


inputted character code, the eight-by-eight bit character


definition and the pixel grid definition boosted by a


factor of 64. Character codes used with program 1 must be


in the range 32 to 127. For example, when code 127 is


inputted, the eight-by-eight binary bit pattern and corres-


ponding eight-by-eight pixel grid definition, magnified by


a factor of 64, for the © character is displayed, figure 3.


[Actually, figure 3 is another character - see later in the


article. There was no figure for ©.


The program in question is on the TZX which goes with this


article, under the name of "Grid".]


Program 1 may be modified so that you can obtain a dis-


play of the eight-by0-eight bit pattern and pixel-grid


definition of a character entered via the keyboard. In this


case the keyboard-entered character is converted to the


corresponding code using the Inkey$ function. To do this


you must change lines 5, 12 and 30 to


5 IF INKEY$="" THEN GO TO 5


12 PRINT AT 5,6;"The character is ";z$


30 LET j=15615+8*(CODE z$-32)+x


and insert the additional lines of program


3 PAUSE 10


10 LET z$=INKEY$


[The result is also on the TZX, as "Grid-inkey".]


With the modified form of the program you cannot input


all the characters with codes in the range 32 to 127 inclu-


sive. For example the } character - code 125 - is excluded.


But the modified form of the program does allow you to


enter most characters directly without looking up the cha-


racter codes in the character set summary.


The characters stored in ROM cannot be changed, and


therefore if you require to display additional characters,


you may define them and store them in random access memory


- RAM.


It is possible to define up to 21 user-defined characters


which require 168 bytes of memory, and the area of RAM


reserved for them has the address range 32600 to 32767 in


the 16K Spectrum and the address range 65368 to 65535 in


the 48K Spectrum.


The address of the first byte in memory of any of the


user-defined graphic in characters may be determined by


using the Usr finction in the form


PRINT USR "user defined graphic symbol"


and the computer outputs to the screen the appropriate


address. For example


PRINT USR "D"


outputs the address 32624 for the 16K Spectrum, and 65392


for the 48K Spectrum.


The area of RAM used for the user-defined characters is


initialised, at switch-on, with data bytes which define the


alphabetic characters A to U. The eight bytes of any of


these characters are stored in the memory from address


Addr+8*(code-144) to address Addr+7+8*(code-144), where


code has a value in the range A to U respectively, as given


in the character set summary. Addr is equal to 32600 for


the 16K Spectrum and it is equal to 65368 for the 48K


Spectrum.


For example, in the 16K Spectrum, if you examine the


content of the eight bytes of memory with addresses from


32672 to 32679 inclusive, you will see that the J character


in binary form as


Memory Memory


address contents


32672 00000000


32673 00000010


32674 00000010


32675 00000010


32676 01000010


32677 01000010


32678 00111100


32679 00000000


A simple way of proving this to yourself is to run Program


1 with line 30 changed to


30 LET j=32599+8*(code-144)+x 16K


or


30 LET j=65367+8*(code-144)+x 48K


[or, for either, 30 LET j=USR "a"-1+8*(code-144)+x.]


[The next line was corrupted - it showed a duplicate of


a line further on in the article. My best guess at what was


intended is:]


[These default user-defined characters will] probably be


of very little use to you because the same characters, A to


U, exist as part of the character set stored in ROM. How-


ever, because the user-defined characters are stored in RAM


you can generate your own character by writing the appro-


priate eight bytes into any of the 21 user-defined graphics


memory areas. For example, to obtain a character of the


form shown in figure 3 [which, this time, is correct] you


must first define the bit pattern of each byte of the


character. In this example we have:


00001000


00010100


00100010


01000001


00100010


00010100


00001000


00000000


Let us assume that this character is to be identified as


the user-defined character D, then we must store the above


bytes in memory addresses Addr to Addr+7, where Addr is


equal to 32624 for the 16K Spectrum and it is equal to


65392 for the 48K Spectrum. We can do this using


POKE Addr , BIN 00001000


POKE Addr+1, BIN 00010100


| | | |


| | | |


| | | |


| | | |


POKE Addr+7, BIN 00000000


To examine the binary bit pattern and pixel-grid definition


of this defined character we may use program 1 with line 30


changed to


30 LET j=32599+8*(code-144)+x 16K Spectrum


or


30 LET j=65367+8*(code-144)+x 48K Spectrum


[or, vide supra.]


To include a user-defined graphic character in a Basic


program you enter graphics mode and access the character


by pressing the appropriate alphabetic symbol key A to U.


For the above example to display the diamond character at


the centre of the screen you enter


PRINT AT 10,15;"÷"


Where ÷ is obtained by putting the computer in graphics


mode and pressing the D key [and in the article was a


diamond shape, but that character is not in the ANSI


charset, so it can't be represented in this text file.]


This character is used in the simple game shown in program


2, in which treasure has been randomly and invisibly buried


in one character square of the screen display. The treasure


is found by moving the character upwards or downwards by


pressing the u or d key respectively, and left or right by


pressing the L or R key, until the ÷ character is on the


character square where the treasure has been buried. The


number of moves taken to find the treasure is then dis-


played. [This is on the TZX as "Game".]


The eight-by-eight pixel grid pattern enables 2 ^ 8 ^ 8


(1.8446744E+19) different characters to be defined, however


you can only have 21 user-defined characters. To create


more characters you can use the Over 1 statement, which


permits one character to overlay another character, but


note that coincident inked pixels are written as Paper.





___________________________________________________________





COPY RETURN CLEAR DRAW CLS IF R


ANDOMIZE SAVE RUN PLOT PRINT POK


E NEXT PAUSE LET LIST LOAD INPUT


GO SUB GO TO FOR REM DIM CONTIN


UE BORDER NEW RESTORE DATA READ


STOP LLIST LPRINT OUT OVER INVER


SE BRIGHT FLASH PAPER INK CIRCLE


BEEP VERIFY MERGE CLOSE # OPEN


# ERASE MOVE FORMAT CAT DEF FN S


TEP TO THEN LINE <>>=<= AND OR B


IN NOT CHR$ STR$ USR IN PEEK ABS


SGN SQR INT EXP LN ATN ACS ASN


TAN COS SIN LEN VAL CODE VAL$ TA


B AT ATTR SCREEN$ POINT FN PIINK


EY$RNDUTSRQPONMLKJIHGFEDCBA¤¤¤¤¤


¤¤¤¤¤¤¤¤¤¤ ©~}|{zyxwvutsrqponmlk


jihgfedcba£_^]\[ZYXWVUTSRQPONMLK


JIHGFEDCBA@?>=<;:9876543210/.-,+


*)('&%#"!





Figure 1.


[The ¤ characters represent block graphics, which are


absent from the ANSI character set - in fact, most of


them aren't even in Unicode.]


___________________________________________________________





Figure 2.


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Figure 3.


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