THE PROGRAM given here can be used with the Pixie version of the Elf microcomputer to create animation graphics using only the original 256 bytes of memory. The interrupt routine uses the same timing as described in previous Elf articles. However, a counter has been added to this routine, and we load the refresh address into R0 from R4. The main line of the program has been completely rewritten and contains shift, roll, and INPUT switch read routines.

The shift routine shiftes 16 lines of the display to the right one bit at a time; bits shifted off the rightmost byte are shifted back onto the display in the [43]

LOC  COSMAC CODE  LNNO   SOURCE LINE

                  1      .. AN 1802 ANIMATION PROGRAM by E. DEVEAUX
                  2      ..
78                3      BEGSFT=#78  .. ADDRESS OF FIRST LINE SHIFTED.
                  4      ..
                  5      .. THIS PROGRAM PROVIDES VARIABLE SPEED
                  6      .. ANIMATION OF THE IMAGE LOCATED AT #78 to
                  7      .. #F7 IN MEMORY.
                  8      .. SPEED CONTROL IS PROVIDED BY INPUT SWITCHES.
D0   90           9                  GHI R0      ..ZERO HIGH ORDER OF
01   B1           10                 PHI R1      ..R1 R2 R3.
02   B2           11                 PHI R2
03   B3           12                 PHI R3
04   B4           13                 PHI R4      ..R4 POINTS TO REFRESH
05   A4           14                 PLO R4         ADDRESS
06   F816         15                 LDI A.0(INTRPT)
08   A1           16                 PLO R1
09   F813         17                 LDI A.0(STACK)
0B   A2           18                 PLO R2
0C   F831         19                 LDI A.0(MAIN)
0E   A3           20                 PLO R3
0F   D3           21                 SEP R3      ..GO TO MAIN_LINE
10   01020300     22                 DC#01020300 ..STACK AREA
13                23     STACK =*-1
                  24     ..
                  25     ..THIS PROGRAM USES A MODIFIED VERSION
                  26     ..OF THE INTERRUPT ROUTINE THAT APPEARED
                  27     ..IN COSMAC ELF PART 4.
                  28     ..
                  29     ..A SHIFT ROUTINE HAS BEEN ADDED THAT MOVES THE
                  30     .. STARSHIP FROM LEFT TO RIGHT ACROSS THE CRT.
                  31     ..
14   72           32       RETURN,LDXA
15   70           33              RET            ..CYCLES
16   22           35       INTRPT,DEC   R2       .. 2
17   78           36              SAV            ..4 R5 COUNTS REFRESH
18   22           37              DEC   R2       ..6 CYCLES, USED TO
19   52           38              STR   R2       ..8 DETERMINE WHEN TO
1A   15           39              INC   R5       ..10 SHIFT /ROLL.
1B   C4           40              NOP            ..13
1C   94           41              GHI   R4       ..15 R4 TO R0