diamond pattern (6502 assembly) 1.0
Author: David Payne
Category: Christmas Challenge 2023
System: BBC Micro
Language: 6502 assembly
Len source code: 284 bytes
Len exe file: 32 bytes
Len code only: 32 bytes
Instructions:
Save the VCCC.ssd file on your local PC
From the https://bbc.godbolt.org website
click on Discs
From examples or local
Choose file
select the saved VCCC.ssd file
enter *RUN VCCC23
Description:
The diamond pattern is 19x19 characters (0 to 18 in the screen coordinates)
the x coordinate is translated into a number from the below table (e.g. x=0 -> 3)
3210123210123210123
the y coordinate is translated into a number from the above table (e.g. y=1 -> 2)
when one of the numbers is 3 and the other one is 0
OR when one of the numbers is 2 and the other one is 1
output an asterisk else output a space
or more simply...
when the 2 numbers add up to 3
output an asterisk else output a space
the program consists of an outer y loop and an inner x loop, both loop 19 times
at the end of the x loop a new line is output
a lookup table is used to get the numbers for x and y
to simplify the code, rather than the rule being the 2 numbers add up to 3, the following rule is used instead:
the 2 numbers when exclusive ORed together equal 42
42 being the ASCII code for an asterisk
the 1st 2 entries in the lookup table are in fact the last 2 bytes of the code, which saves 2 bytes
these are:
229
96
which means that the next 2 table entries become:
74 (96 EOR 42)
207 (229 EOR 42)
and the rest of the table follows the established pattern (the next few entries being 74, 96, 229, etc.)
Also, as the table repeats after every 6 entries, only the first 6 entries are needed initially;
the rest of the table is built up as the program runs
Comments:
To produce the VCCC23 executable:
Copy and paste the SOURCE.txt file into jsbeeb (https://bbc.godbolt.org)
enter RUN
enter *SAVE VCCC23 53 73
10FORI%=0TO2STEP2
20P%=&53
30[OPT I%
40.loop1 LDX #data-2
50.loop2 LDA 0,X
60STA 6,X
70.eor EOR data-2
80CMP #42
90BEQ skip
100LDA #32
110.skip JSR &FFEE
120INX
130BPL loop2
140JSR &FFE7
150INC eor+1
160BPL loop1
170RTS
180.data EQUB 74
190EQUB 207
200EQUB 74
210EQUB 96
220]
230NEXT