130 lines
5.0 KiB
C
130 lines
5.0 KiB
C
/* ------------------------------------------------------------------------
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System : Manx Aztec C65 Version 3.2b
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MS-DOS cross-development environment
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Platform : Apple IIe 128K PRODOS 8
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Program : dhrcalc.c
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Description : G2 Library Routine
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Double Hi-Res 140 x 192 x 16 color
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helper routine for pixel drawing functions.
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inputs: x co-ordinate 0-139 and color 0-15
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outputs required information for pixel drawing
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to static structure used by pixel drawing functions.
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range checking and things of that nature
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are done outside of here.
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values that are not used by drawing function logic
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are not set. said logic depends on the particular
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bank position in the framebuffer and whether or not
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that position is using main and/or auxiliary memory.
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Written by : Bill Buckels
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Date Written : January 2013
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Revision : 1.0 First Release
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Licence : You may use this code for whatever you wish as long
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as you agree that Bill Buckels has no warranty or
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liability obligations whatsoever from said use.
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------------------------------------------------------------------------ */
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#include <dhrpixel.h>
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extern struct dhrpixel dhrx;
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extern unsigned char dhapix[16][7];
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extern unsigned char dhmpix[16][7];
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dhrcalc(x, drawcolor)
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int x, drawcolor;
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{
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dhrx.pattern = (x%7); /* 7 pixel color pattern */
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/* default - set the initial offset into the framebuffer for each byte pair in
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both memory banks to the first byte to avoid excessive mult and div
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instructions bulking-up this object module with repetitive code */
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dhrx.xaux = dhrx.xmain = (x/7) * 2; /* assign offset for pointer addresses */
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/* Summary of AND and OR bitwise operators
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bit a bit b a & b a | b
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0 0 0 0
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0 1 0 1
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1 0 0 1
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1 1 1 1
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Bitmasking can be a confusing business in practice only because
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the code to do so is barely man-readable and even less man-writable...
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but theoretically straight-forward and quite efficient:
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1. Bitwise ANDing a binary 1 will preserve the bit and
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Bitwise ANDing a binary 0 will erase the bit.
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2. Combining 2 values by inclusively ORing them together works
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fantastically if the areas to be combined have been mutually zeroed
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in the complementary bits of the other value by using a bitwise AND.
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*/
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/* the following switch statement provides the dhrpixel structure with all the data necessary
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for the calling function to plot a color in a more or less data-driven abstract manner...
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review dhrvline to see how the rest of this is done */
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/* assign color bitmap for inclusive OR from 7 pixel level color bitmap patterns
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for either auxiliary or main memory respectively from the dhapix or dhmpix arrays. */
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/* provide mask value to erase target framebuffer area before including color bitmap */
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/* increment framebuffer pointer offset to second byte in byte pair if needed */
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switch(dhrx.pattern)
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{
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case 0:
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dhrx.caux = dhapix[drawcolor][0]; /* 00001111 assign 4 bits */
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dhrx.mska = 0x70; /* 01110000 erase 4 bits */
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break;
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case 1:
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dhrx.caux = dhapix[drawcolor][1]; /* 01110000 assign 3 bits */
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dhrx.mska = 0x0f; /* 00001111 erase 3 bits */
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dhrx.cmain = dhmpix[drawcolor][1]; /* 00000001 assign 1 bit */
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dhrx.mskm = 0x7e; /* 01111110 erase 1 bit */
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break;
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case 2:
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dhrx.cmain = dhmpix[drawcolor][2]; /* 00011110 assign 4 bits */
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dhrx.mskm = 0x61; /* 01100001 erase 4 bits */
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break;
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case 3:
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dhrx.cmain = dhmpix[drawcolor][3]; /* 01100000 assign 2 bits */
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dhrx.mskm = 0x1f; /* 00011111 erase 2 bits */
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/* the byte pair splits here at meridian pixel 3 of 7 with the first
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half of the pixel in the high nibble of the first pair in the main memory bank
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and the second half of the pixel in the low nibble of the second pair in
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the auxiliary memory bank */
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dhrx.xaux++; /* second byte in auxiliary memory byte pair */
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dhrx.caux = dhapix[drawcolor][3]; /* 00000011 assign 2 bits */
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dhrx.mska = 0x7c; /* 01111100 erase 2 bits */
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break;
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case 4:
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dhrx.xaux++ ; /* second byte in auxiliary memory byte pair */
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dhrx.caux = dhapix[drawcolor][4]; /* 00111100 assign 4 bits */
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dhrx.mska = 0x43; /* 01000011 erase 4 bits */
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break;
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case 5:
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dhrx.xaux++; /* second byte in auxiliary memory byte pair */
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dhrx.caux = dhapix[drawcolor][5]; /* 01000000 assign 1 bit */
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dhrx.mska = 0x3f; /* 00111111 erase 1 bit */
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dhrx.xmain++; /* second byte in main memory byte pair */
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dhrx.cmain = dhmpix[drawcolor][5]; /* 00000111 assign 3 bits */
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dhrx.mskm = 0x78; /* 01111000 erase 3 bits */
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break;
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case 6:
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dhrx.xmain++; /* second byte in main memory byte pair */
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dhrx.cmain = dhmpix[drawcolor][6]; /* 01111000 assign 4 bits */
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dhrx.mskm = 0x07; /* 00000111 erase 4 bits */
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}
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return;
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} |