Move #09 02 августа 1997

Программирование

algorithm - On addressing Spectrum screen.

                ALGORITHMS

(C) Ars


    In this section we continue
publish useful procedure. This
times will discuss the various routines
associated with the output on screen. If you have
encounter any comments, please
us. And also can you suggest to
discussion of his version of the algorithms
contact the editor MOVE.

    Part of the procedures described below,
wrote Rui Ribeiro - author WSpecem -
Spectrum emulator for Windows.


    Primarily on addressing
Spectrum screen. Despite
some inconsistencies in his
structure, the address of any byte-screen
Region (# 4000 .. # 57FF) can be given
scheme:


          010S SLLL RRRB BBBB

where: S - Section, or three (0 .. 2);

     L - number of lines character (0 .. 7)

     R - The vertical position within

         Section (0 .. 7);

     B - column number (0 .. 31).

Coordinates of any point (x, y) screen in these
notations have the structure:

y - 11,111,111 x - 11 111 111

     S R L B bit number

y = 0 .. 191 x = 0 .. 255

And here's how you describe the address of the screen
following formula:

 adr (x, y) = 16384 +2048 * INT (L / 8) +
 +32 * (L-8 * INT (L / 8)) +256 * R + C

Now - the main procedure.

1) As from the coordinates of the points to get it
display the address and number of pixels per byte.

Input: BC = yx
Output: HL = screen address, A = bit

PixAdr1 LD A, B

         AND A

         RRA

         SCF

         RRA

         AND A

         RRA

         XOR B

         AND # F8

         XOR B

         LD H, A

         LD A, C

         RLCA

         RLCA

         RLCA

         XOR B

         AND # C7

         XOR B

         RLCA

         RLCA

         LD L, A

         LD A, C

         AND 7

         RET

2) The same problem, but instead coordinates x
given column number.

Input: B = pixel line number (0 .. 191)

        C = column number (0 .. 31)
Output: HL = screen address

PixAdr2 LD A, B

         AND A

         RRA

         SCF

         RRA

         AND A

         RRA

         XOR B

         AND # F8

         XOR B

         LD H, A

         LD A, B

         RLCA

         RLCA

         AND # E0

         OR C

         LD L, A

         RET

The position of any text characters on
screen is given by the following scheme:

Y - 000 11111 X - 000 11 111

        S R B

Y = 0 .. 23 X = 0 .. 31

3) As of the character position to get
display address.

Input: BC = YX
Output: HL = screen address

ChrAdr1 LD A, B

         AND # 18

         OR # 40

         LD H, A

         LD A, B

         AND 7

         RRCA

         RRCA

         RRCA

         ADD A, C

         LD L, A

         RET

Or another option:

ChrAdr2 LD A, B

         AND # 18

         OR # 40

         LD H, A

         LD A, B

         RRCA

         RRCA

         RRCA

         AND # E0

         OR C

         LD L, A

         RET

The following procedures relate to the text output

4) The descent down to the symbol.

Input: HL = screen address
Output: HL = down char screen address

DownChr LD A, L

         ADD A, 32

         LD L, A

         RET NC

         LD A, H

         ADD A, 8

         LD H, A

         RET

5) Climb up the symbol.

Input: HL = screen address
Output: HL = upper char screen address

UpChar LD A, L

         SUB 32

         LD L, A

         RET NC

         LD A, H

         SUB 8

         LD H, A

         RET

6) Go to the next character to
a display address.

Input: HL = screen address
Output: HL = next char screen address

NextChr RR H

         RR H

         RR H

         INC HL

         RL H

         RL H

         RL H

         RET

7) Go to the previous character in
a display address.

Input: HL = screen address
Output: HL = previous char screen address

PrevChr RR H

         RR H

         RR H

         DEC HL

         RL H

         RL H

         RL H

         RET

Information obtained from Web-page:
http://www.atlantic.net/ ~ adansby / plot.html

(Continued in the next issue.)