Using Pixel Graphics From Assembler
By G. N. Evans
The Basic ROM on the Nascom contains plenty of useful routines for the
assembler programmer. The problem is finding out where they are and, more
importantly, how to drive them. I have discovered most of the useful routines, and I
am using many of them in a Basic Compiler which I am currently writing.
Below I have given details on how to make use of the pixel graphics routines.
Should you try to access a non-existent point, then Basic would take over, issue an
error message, and then remain in Basic. To prevent this I have written a routine,
CHECK, which tests the validity of the co-ordinates; the carry flag is set on return if
they are not valid. The routine also converts, the y co-ordinates so that y = 0
corresponds to the bottom of the screen and y = 47 to the top, which is much more
logical and convenient than Microsoft’s ordering of the y axis.
Here is an example of the use of the routines to draw a vertical line through the
centre of the screen:
| 11 00 18 | VERT | LD DE, £1800 | POINT (24,0) |
| D5 | NEXT | PUSH DE | Save current point |
| CD 1F 80 | | CALL CHECK | Legal co-ordinates? |
| 38 0C | | JR C, ERROR | If not, jump |
| CD 00 80 | | CALL SETP | Set point |
| D1 | | POP DE | Recover current point |
| 1C | | INC E | Next point up screen |
| 7B | | LD A, E | Transfer to accumulator |
| FE 30 | | CP 48 | Has whole line been set? |
| 20 F0 | | JR NZ, NEXT | Repeat if not |
| DF 5B | | SCAL £5B | Repeat to monitor, etc. |
| ERROR | … | Put error routine here |
Routines To Use The Pixel Graphics
| | | ORG £8000 | |
| | | ADDRESSES IN BASIC ROM |
| | BCSPOS | EQU £FF31 | |
| | BSET | EQU £FF43 | |
| | BRESET | EQU £FF58 | |
| | BPOINT | EQU £FFED | |
|
SET (D, E) |
| 8000 | CD1680 | SETP | CALL CSPOS | Calculate screen posn. |
| 8003 | CD43FF | | CALL BSET | Use Basic ROM |
| 8006 | C9 | | RET | |
| |
All register corrupted (except alternate set) |