juliem_bbc_6502_vc3-2024.zip
----------------------------
List of files:
file_id.diz - this file
juliem_bbc_6502_vc3-2024.ssd - BBC micro disc image
listing.png - image of (partial) source listing
output.png - image of output
README.md - instructions, annotated assembler source
source.bas - BBC Micro BASIC / assembler source
(detokenised, readable on host)
present - the binary executable
LOAD = &0040 EXEC = &0040
Author: Julie Kirsty Louise Montoya <bluerizlagirl@gmail.com>
Category: Christmas Challenge
System: BBC Micro
Language: BBC BASIC / 6502 Assembler (built-in BASIC assembler)
Len source code: 882 bytes (tokenised on target system)
1043 bytes (detokenised on host system)
Len exe file: 78 bytes
Len code only: 78 bytes
Instructions:
Install BeebEm or another suitable BBC emulator, if you have not already ;)
Start emulator. NB: Make sure Econet is disabled!
Load the disc image "juliem_bbc_6502_vc3-2024.ssd" in drive 0
Make sure the disc is NOT write-protected
Enter the following command to assemble the machine code:
CHAIN"SOURCE"
Press SHIFT to scroll the screen when it becomes full.
There will now be a new file on the disc called PRESENT
Type the following to run the assembled code:
*PRESENT
Description:
SOURCE is a BASIC program which uses the BBC Micro's built-in assembler
to assemble a 6502 machine code program, which draws a present identical
to the pattern given in the programming challenge, as near to the centre
of the screen as possible.
\O/
+--------+--------+
! ! !
! ! !
! ! !
! ! !
! ! !
! ! !
! ! !
! ! !
+--------+--------+
! ! !
! ! !
! ! !
! ! !
! ! !
! ! !
! ! !
! ! !
+--------+--------+
The program first prints a series of control codes to select a suitable
screen MODE and position the cursor for the bow; prints the characters
\, O and /; and finally prints control codes to return the cursor to the
beginning of the line and move down.
To draw the main body of the present, we rely on the diagonal symmetry
of the shape. A single table can then be used to look up whether or not
we need to draw anything. The table holds the following values:
1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1
At each position we double the row value (giving 2 where a horizontal
line is required), add the column value (giving 1 where a vertical
line is required, or 3 where two lines meet), and use this to select a
character to print to the screen.
Since the table repeats after nine values, we only need to store those
first nine values; and then we can use the "cartoon railroad" technique
to extend the table as we read it, like laying down fresh track in front
of a moving train.
To take advantage of the 6502's special "short" mode, the whole program
sits in zero page. The locations to which it loads are mostly safe
(&70-8F are guaranteed not to be used by BBC BASIC, &50-&6F don't seem
to be used by BBC BASIC and &40-&4F will be overwritten by BBC BASIC
but only when the program has finished). The program will overwite
another 19 bytes of memory and this is supposed to be for the use of
filing systems, but it did not crash my emulator. If you run this on
real hardware, do not have any irreplaceable discs in the drives!