STAR

Author: regregex
Category: Christmas Challenge
System: BBC Micro A/B/Master, Acorn Electron
Language: BBC BASIC
Len source code: 115 bytes
Len exe file:     87 bytes
Len code only:    87 bytes
Instructions:
  Install and open BeebEm ( http://www.mkw.me.uk/beebem/ ).
  Click File -> Disc Options -> New Disc 0, enter star.ssd
  Click Edit -> Import Files to Disc -> Disc 0, select star.inf
  At the BASIC prompt, type CHAIN"STAR" and press RETURN.

  To reproduce the screenshots:
  Click Hardware -> BBC Model -> BBC Master 128
  Enter the following:
  *INSERT 9
  *CONFIGURE FILE 9
  *CONFIGURE MODE 4
  Press Ctrl+F12 (CTRL+BREAK), releasing Ctrl last.
  Then follow the instructions above.

Description: The program runs under BBC BASIC version 1 and up.
First it clears the screen, turns the cursor off (with five
little-endian words to the VDU driver) and backspaces; with the cursor
in the rightmost column, the program loops through the 17 rows of the
shape and each column of the screen, printing an asterisk or space in
each position.  Finally it waits for a keypress, then exits.

FOR statements evaluate their expressions once, so the inner loop prints
one row of characters to begin and end with the cursor in the rightmost
column.
The character is chosen by XORing 42 either with 0 to yield 42, the
ASCII code for "*", or with 10 to yield 32, the ASCII code for a space.
The Boolean expression in parentheses evaluates TRUE when the point has
a grid distance more than 4 from the nearest diagonal, OR when both
coordinates exceed +/-4.  As FALSE is all zeroes in binary and TRUE is
all ones, the first AND in the expression masks the Boolean result to
0 or 10.  The final EOR produces the character code for the VDU
statement to print.

Comments:
Accommodates all BBC Micro display MODEs and text windows down to 18
columns wide (e.g. VDU 28,0,24,17,0).
Putting EOR after the bracketed expression separates it from a number,
which would parse the E as an exponent.  This saves a space before EOR.
GET returns a number (the ASCII code of the key pressed) which IF,
having no THEN or ELSE clause, discards.