Re: A82: Find Pixel Question

To: assembly-82@lists.ticalc.org
Subject: Re: A82: Find Pixel Question
From: ADAMMAN106 <ADAMMAN106@aol.com>
Date: Sun, 17 May 1998 21:17:21 EDT
Delivered-To: assembly-82-outgoing@towerguard.unix.edu.sollentuna.se
Reply-To: assembly-82@lists.ticalc.org

In a message dated 98-05-17 13:49:49 EDT, you write:

> Hello,
>      Would someone please tell me how exactly Find_Pixel works [the input,
>  the output, etc.] because I am going to attempt to make a Find_Pixel
routine
>  for the 83 so I can port some games...  Any help would be appreciated.
>                                                                             
>                          -Ahmed

instead of FIND_PIXEL, use this (from FLine):

        xor     A				;*** A is provided for overflows (ld a, 0)
        ld      C, E				;*** this section multiplies E by 16
        sla     C				;*** changed to multiply E by 12 by Adamman
        sla     C				;*** used as a fast substitute for FIND_PIXEL
        rla

        ld      B, C				;*** added by Adamman

        sla     C
        rla

        push    AF				;*** added by Adamman
        ld      A, B				;*** added by Adamman
        add     A, C				;*** added by Adamman
        ld      C, A				;*** added by Adamman
        pop     AF				;*** added by Adamman

;        sla     C				;*** removed by Adamman
;        rla					;*** removed by Adamman
        ld      B, A
        ld      HL, GRAPH_MEM			;*** changed from ld hl, VIDEO_MEM by Adamman
        add     HL, BC

        ld      B, 0				;*** this section divides D by 8
        ld      C, D
        srl     C
        srl     C
        srl     C

        add     HL, BC				;*** now we have the byte we are going to use
        ld      (DRAW_BYTE), HL		;*** but we still need the bit!

;*** this section removed:
;        ld      HL, (PROGRAM_ADDR)  ;could be optimized w/ a substitution tbl
;        ld      BC, BIT_TABLE
;        add     HL, BC

;*** replaced with this:
        ld      HL, BIT_TABLE
;*** end replacement

        ld      A, D				;*** this section determines the bit to change
        and     00000111b
        ld      C, A
        ld      B, 0
        add     HL, BC
        ld      A, (HL)
        ld      (BIT_MASK), A

BIT_TABLE:
.db 10000000b,01000000b,00100000b,00010000b
.db 00001000b,00000100b,00000010b,00000001b

Input is point (d,e) as a coordinate in pixels, (0,0) being the TOP LEFT HAND
corner of the screen (unlike FIND_PIXEL which uses the bottom left).  Of
course change "ld hl, GRAPH_MEM" to whatever buffer you want it to record to.
output would be (BIT_MASK), one byte and the bit to change and (CHANGE_BYTE),
two bytes and the byte to change in the graph mem.  in FIND_PIXEL, input is
(b,c), a is the bit to change, and hl is the byte to change, which must be
added to GRAPH_MEM to get the address in the graph mem.
Resident experts (Dines, Crash Man, Barubary), please make sure this is right!

~Adamman