170 lines
7.3 KiB
C++
170 lines
7.3 KiB
C++
/*
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epple2
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Copyright (C) 2008 by Christopher A. Mosher <cmosher01@gmail.com>
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY, without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>.
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*/
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#ifndef E2CONST_H
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#define E2CONST_H
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class E2Const
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{
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public:
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/*
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The NTSC standard defines the field rate as 60 fields per second. The number 60
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is based on the USA AC current frequency of 60 Hz. This, in turn, was based on the
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clock standard (60 seconds per minute and 60 minutes per hour).
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*/
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static const int NTSC_FIELD_HZ = 60;
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/*
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The NTSC standard defines 525 lines per frame, which was chosen to be a multiple
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of a small number of standard tubes at the time, to produce a rate between RCA's
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recommended 441 (used by NBC) and Philco's suggested 600-800 lines.
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*/
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static const int NTSC_LINES_PER_FRAME = 3*5*5*7;
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/*
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When color was added to the NTSC signal, studies by General Electric showed that
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minimum interference was achieved using a subcarrier frequency 455 times the field
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rate, which can also be obtained using standard tubes.
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*/
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static const int NTSC_COLOR_MULTIPLE = 5*7*13;
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/*
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Adding color to NTSC also required slowing down the frame rate, by dropping one
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field after every 1000.
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*/
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static const int NTSC_COLOR_DROP_FIELD = 1000;
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/*
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Calculate the color sub-channel rate, times 4.
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This will be the (approximate) Hz of the "14M"
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crystal oscillator in the Apple ][.
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14318181.818181818... Hz rounds to 14318182 Hz
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U.A.II, p.3-2
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*/
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static const int CRYSTAL_HZ = (int)(1.0F*NTSC_FIELD_HZ * NTSC_LINES_PER_FRAME * NTSC_COLOR_MULTIPLE * NTSC_COLOR_DROP_FIELD / (NTSC_COLOR_DROP_FIELD+1));
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/*
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U.A.II, p. 3-3
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Normal 6502 cycle == 14 crystal periods
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Long 6502 cycle == 16 crystal periods
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*/
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static const int CRYSTAL_CYCLES_PER_CPU_CYCLE = 14;
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static const int EXTRA_CRYSTAL_CYCLES_PER_CPU_LONG_CYCLE = 2;
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/*
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65 bytes per row (64 normal CPU cycles plus one long CPU cycle)
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*/
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static const int BYTES_PER_ROW = (int)((NTSC_COLOR_DROP_FIELD+1)*1.0F*CRYSTAL_HZ/(NTSC_FIELD_HZ/2*NTSC_COLOR_DROP_FIELD*NTSC_LINES_PER_FRAME*CRYSTAL_CYCLES_PER_CPU_CYCLE));
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static const int HORIZ_CYCLES = BYTES_PER_ROW;
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/*
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U.A.II, p. 3-2, "composite frequency... 1.0205 MHz"
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Actually 1020484 Hz.
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*/
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static const int AVG_CPU_HZ = (int)((1.0F*CRYSTAL_HZ*HORIZ_CYCLES)/(CRYSTAL_CYCLES_PER_CPU_CYCLE*HORIZ_CYCLES+EXTRA_CRYSTAL_CYCLES_PER_CPU_LONG_CYCLE));
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/*
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A normal NTSC field is 262.5 lines (half of a full frame's 525 lines).
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The Apple rounds this down to 262 lines.
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*/
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static const int NTSC_WHOLE_LINES_PER_FIELD = NTSC_LINES_PER_FRAME/2;
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static const int BYTES_PER_FIELD = BYTES_PER_ROW*NTSC_WHOLE_LINES_PER_FIELD;
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// exactly 1 million
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static const int MEGA = 1000000;
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static const int VISIBLE_BITS_PER_BYTE = 7;
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static const int VISIBLE_LINES_PER_CHARACTER = 8;
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/*
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* 1000+1 seconds 2 fields 1 frame 1000000 microseconds 63 50
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* total horizontal line period = -------------- * -------- * ------------- * -------------------- = ( -- + -- ) microseconds per line
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* 60*1000 fields 1 frame 3*5*5*7 lines 1 second 90
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*
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* 10 81
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* horizontal blanking period = (1.5+4.7+.6+2.5+1.6) = 10.9 microseconds per line = ( -- + -- ) microseconds per line
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* 90
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*
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* visible line period = total horizontal line period minus horizontal blanking period =
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*
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* 52 59
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* -- + -- microseconds per line
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* 90
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*
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*
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* To avoid the over-scan area, the Apple ][ uses only the middle 75% of the visible line, or 4739/120 microseconds
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*
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* Apple ][ uses half the clock rate, or 315/44 MHz, to oscillate the video signal.
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*
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* The result is 315/44 MHz * 4739/120 microseconds/line, rounded down, = 282 full pixel spots across the screen.
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* The Apple ][ displays 7 bits per byte hi-res or lo-res, (or 7 pixel-wide characters for text mode), so that
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* gives 282/7, which rounds down to 40 bytes per line.
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*/
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static const int VISIBLE_BYTES_PER_ROW = (int)((((1.0F*(NTSC_COLOR_DROP_FIELD+1)/(NTSC_FIELD_HZ*NTSC_COLOR_DROP_FIELD)*2/NTSC_LINES_PER_FRAME*MEGA)-(1.5+4.7+.6+2.5+1.6)) * 3/4) * (CRYSTAL_HZ/2)) / MEGA / VISIBLE_BITS_PER_BYTE;
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/*
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* NTSC total lines per frame (525) minus unusable lines (19 plus 20) = 486 usable lines
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* To avoid the over-scan area, use the middle 80% of the vertical lines, giving 388 (rounded down) clearly visible lines
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* Apple ][ uses only half the vertical resolution because it doesn't interlace, giving 194.
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* Text characters are 8 pixels tall, so 194/8 rounded down gives 24 text lines.
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* Multiply by 8 to give 192 lines total.
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*/
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static const int VISIBLE_ROWS_PER_FIELD = (NTSC_LINES_PER_FRAME-(20+19)) * 8/10 / 2 /VISIBLE_LINES_PER_CHARACTER*VISIBLE_LINES_PER_CHARACTER;
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static const int BLANKED_BYTES_PER_ROW = BYTES_PER_ROW-VISIBLE_BYTES_PER_ROW;
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static const int VISIBLE_BYTES_PER_FIELD = BYTES_PER_ROW*VISIBLE_ROWS_PER_FIELD;
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static const int SCANNABLE_ROWS = 0x100;
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static const int SCANNABLE_BYTES = SCANNABLE_ROWS*BYTES_PER_ROW;
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static const int RESET_ROWS = NTSC_WHOLE_LINES_PER_FIELD-SCANNABLE_ROWS;
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static const int RESET_BYTES = RESET_ROWS*BYTES_PER_ROW;
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static const int MIXED_TEXT_LINES = 4;
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static const int ROWS_PER_TEXT_LINE = 8;
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static const int MIXED_TEXT_CYCLE = (VISIBLE_ROWS_PER_FIELD-MIXED_TEXT_LINES*ROWS_PER_TEXT_LINE)*BYTES_PER_ROW;
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static int test()
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{
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if (NTSC_FIELD_HZ!=60) return NTSC_FIELD_HZ;
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if (NTSC_LINES_PER_FRAME!=525) return NTSC_LINES_PER_FRAME;
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if (NTSC_COLOR_MULTIPLE!=455) return NTSC_COLOR_MULTIPLE;
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if (NTSC_COLOR_DROP_FIELD!=1000) return NTSC_COLOR_DROP_FIELD;
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if (CRYSTAL_HZ!=14318182) return CRYSTAL_HZ;
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if (BYTES_PER_ROW!=65) return BYTES_PER_ROW;
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if (AVG_CPU_HZ!=1020484) return AVG_CPU_HZ;
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if (BYTES_PER_FIELD!=17030) return BYTES_PER_FIELD;
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if (VISIBLE_BYTES_PER_ROW!=40) return VISIBLE_BYTES_PER_ROW;
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if (VISIBLE_ROWS_PER_FIELD!=192) return VISIBLE_ROWS_PER_FIELD;
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if (RESET_BYTES!=390) return RESET_BYTES;
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if (BLANKED_BYTES_PER_ROW!=25) return BLANKED_BYTES_PER_ROW;
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if (VISIBLE_BYTES_PER_FIELD!=12480) return VISIBLE_BYTES_PER_FIELD;
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if (SCANNABLE_BYTES!=16640) return SCANNABLE_BYTES;
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return -1;
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}
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};
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#endif
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