ARGYLL

Author: regregex
Category: Christmas Challenge
System: BBC Micro A/B/Master, Acorn Electron
Language: 6502 assembler (MASM)
Len source code: 1676 bytes
Len exe file:      49 bytes
Len code only:     49 bytes
Instructions:
  Install and open BeebEm ( http://www.mkw.me.uk/beebem/ ).
  Click File -> Disc Options -> New Disc 0, enter argyll.ssd
  Click Edit -> Import Files to Disc -> Disc 0, select xargyll.inf
  At the BASIC prompt, type */X.ARGYLL 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.
  To show the editor screen, import argyll.inf as above, then enter:
  *EDIT ARGYLL
  Shift+F10 (SHIFT+F0) hides newline markers.

Description: The object code clears the screen, then loops through rows
19 to 1 of the shape and columns 19 to 1 of each row.  At each position
it adds the row number y to the column number x and tests whether:

        x + 127 + y == 0 (mod 6)

If the sum divides then it prints an asterisk, forming the forward
diagonals.  Otherwise it modifies the column value and tests for:

        -x + 63 + y == 0 (mod 6)

If this sum divides, the code branches to print an asterisk otherwise it
inverts the column value again, finds it positive and falls through to
print a space.  The constants are chosen so that a single EOR operation
negates x and modifies the constant term.  The character itself is
created by EORing zero (from a successful division) or 10 (otherwise)
with 42, forming ASCII values 42 or 32 respectively.

At the end of each row, the code calls a standard OS routine to print a
newline.  Once 19 rows are completed, it jumps into the OSRDCH entry
point to read (and discard) a character of input and then return control
to the user.

Comments:
Trailing spaces are printed on each line.
The X.ARGYLL binary is relocatable.