1st October 1995I promised to discuss the making up of screens this issue as I had in mind a program from Electron User which I had converted to a completely Machine Code game and I hope to publish this in the next issue together with an explanation of what I have done.
First, however, are some routines that I have not yet covered and that can be used to make up screens for simple games.
In my first article, I gave a simple routine (uline) for printing a line of the same character. If we use the 'tab' subroutine also given in that article, we can print horizontal or vertical lines of the same character anywhere on the screen. Add these two routines to those already entered:
.horiz JSRtab
INX
CPXlimit
BNEhoriz
RTS
.vert JSTtab
INY
CPYlimit
BNEvert
RTS
Add limit=&86 to the variables at line 40. Then to use the routines, enter in the main loop:-
For a horizontal line:-
LDA# <end col> :STAlimit
LDX# <start col>
LDY# <line>
LDA# <char>
JSR horiz } or JMPhoriz
RTS }
For a vertical line:-
LDA# <end line> :STAlimit
LDX# <column>
LDY# <start line>
LDA# char
JMP vert
Blocks of characters can be printed as in BASIC by entering the data -
EQUS CHR$224+CHR$225+CHR$8+CHR$8+CHR$10+CHR$226+CHR$227
and using the loop:-
.comms2 STX addr
STY addr+1
TAX
LDY#0
.clp LDA (addr),Y
JSR oswrch
INY
DEX
BNE clp
RTS
Whilst the loop (comms) is a little more economical in memory and perhaps better programming, this loop is easier to use as it avoids the necessity to reverse all the data. It is used as before:
LDA# <number of commands>
LDX# <data> MOD256
LDY# <data> DIV256
JMP comms2
Windows can be made up using the data:
Text window:-
EQUB28:EQUB <left>:EQUB <bottom>:EQUB <right>:EQUB <top>
Graphics window:-
EQUB24:EQUW <left>:EQUW <bottom>:EQUW <right>:EQUW <top>
Double height lettering can be printed with this routine:-
.dh STX db: STY db+1
LDA#0:STA &70
.b1 LDY &70:LDA (db),Y
CMP #13:BEQ b3
STA &71:INY:STY &70
LDX #&71:LDY #0
LDA #10:JSR &fff1
LDX #1:LDY #0
.b2 LDA &71,X:STA &CF0,Y
INY:STA &CF0,Y
INY:INX
CPX #9:BNE b2
LDA #254:JSR oswrch
LDA #8:JSR oswrch:LDA #10:JSR oswrch
LDA #255:JSR oswrch
LDA #11:JSR oswrch
JMP b1
.b3 RTS
The variable db=&80 must be added to the variables in line 40. This will hold the address of the STRING to be printed which must have the CR character &0D at the end. The routine is used in the same way as the print routine by:
LDX # <label> MOD256
LDY # <label> DIV256
JMP dh
Note that &70 holds the position in the string of the character being printed and &71 holds the character. The character codes are then split to &72 to &79. These are doubled loaded to &CF0 to &Cff for characters 254 and 255.
The routines given so far should enable you to make up screens for simple games where a clash may be detected by comparing co-ordinates where these are held at say &74 (x, y) and &76 (x1, y1):-
LDAx:CMPx1:BNE no_clash
LDAx+1:CMPx12+1:BNE no_clash
.clash <clash routine>
.no_clash RTS
For arcade games of the Repton type a map is needed as in BASIC and this should be entered to a suitable address for the M/code. The data can be saved separately with other similar files so that they can be loaded into the main game.
To load these, we use the OS routine OSCLI (&FFF7). This is used for all * commands. It needs the X and Y registers to be loaded with the address of the command data as for other routines before being called:
LDX# comm MOD256
LDY# comm DIV256
JMP &FFF7
.comm EQUS "LOAD map <address>"
While it may not be necessary to add the address (if it is saved to the correct address), it can be put in.
I shall explain more about maps and other routines next time.
Richard Dimond, EUG #22
