Home » Archimedes archive » Acorn User » AU 1997-10 A.adf » Extras » Apple][e/!Virtu/Data/Docs/Tech

Apple][e/!Virtu/Data/Docs/Tech

This website contains an archive of files for the Acorn Electron, BBC Micro, Acorn Archimedes, Commodore 16 and Commodore 64 computers, which Dominic Ford has rescued from his private collection of floppy disks and cassettes.

Some of these files were originally commercial releases in the 1980s and 1990s, but they are now widely available online. I assume that copyright over them is no longer being asserted. If you own the copyright and would like files to be removed, please contact me.

Tape/disk: Home » Archimedes archive » Acorn User » AU 1997-10 A.adf » Extras
Filename: Apple][e/!Virtu/Data/Docs/Tech
Read OK: ✔
File size: 1FF4 bytes
Load address: 0000
Exec address: 0000
File contents
Technical Notes
===============

Apple2eROM Image
~~~~~~~~~~~~~~~~
    The Apple2eROM image file should be the same format as used by the AppleWin
emulator (for the IBM PC) and is as follows:

+&0000 External I/O ROM ($C000-$CFFF)
+&1000 Internal I/O ROM ($C000-$CFFF)
+&2000 Monitor ROM      ($D000-$FFFF)

Processor Emulation
~~~~~~~~~~~~~~~~~~~
    The NMOS 6502 had several problems which were rectified in later models in the
series. Virtu accurately emulates the known bugs found in the original:

- jump indirect wraps the effective address at a page boundary
- invalid flags after an arithmetic operation in decimal mode

    Another problem with the NMOS 6502 was the operation of undefined or
'undocumented' opcodes which were executable, but not legal instructions. Most of
these opcodes result in the CPU attempting to effectively execute two legal
instructions at once, and thus produce obscure, and even amusing operations. Virtu
implements these opcodes as multi-byte NOP instructions as per the operand length of
the opcodes in a Rockwell R6502. The cycle time used is the maximum of the times for
each of the two legal instructions that comprise the undefined instruction.

    The 65C02 amends all the problems mentioned above; however some older software
may rely on this erroneous functionality, thus Virtu's capability to switch between
the two processors becomes desirable. In particular, the indirect jump and undefined
instructions were sometimes used to make copy protection routines hard to follow,
and some games may rely on the incorrect decimal arithmetic result flags for score-
keeping. Note also that although the 65C02 defines the 6502's undefined opcodes as
either new instructions or multi-byte NOPs, the number of bytes consumed (ie operand
length) differs.

Sound Emulation
~~~~~~~~~~~~~~~
    Memory accesses in the range $C030 to $C03F, both reads and writes, generate a
speaker click on an Apple ][. Virtu buffers such accesses and the VVoice module uses
this information to generate square-wave pulses (scaled by volume) for the sound
system. Intense speaker activity at access periods of around 48 microseconds or less
would 'bog down' the sound emulation, since this is the output sample period for
the single sound channel used. To prevent this, accesses are ignored if they occur
at intervals of around 48 microseconds or less. Although a smaller output period
would improve sound resolution, it significantly increases overheads; 48us is a
compromise that gives a fuller sound for the average case.

Disk ][ Drive Emulation
~~~~~~~~~~~~~~~~~~~~~~~
    Virtu currently supports only one type of Apple ][ disk image, the DSK (from the
MSDOS ".dsk" suffix), or DOS order (DO) disk. This image format has no header
information but is simply the sector data itself, ordered by DOS sector number. Thus
for track 0 the sectors 0, 1, 2 ... F are found at offsets $000, $100, $200 ... $F00
in the DSK. This ordering comes in handy when editing DOS disks with a hex file
editor. For reference, the following table translates physical (numbering as encoded
in sector address fields) to DOS sectors and vice versa.

Physical:  0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F
DOS:       0  7  E  6  D  5  C  4  B  3  A  2  9  1  8  F

DOS:       0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F
Physical:  0  D  B  9  7  5  3  1  E  C  A  8  6  4  2  F

    The track format emulated is a standard DOS 3.3 16 sector track with 4&4-
encoded sector addressing and 6&2-encoded sector data. Although each sector has its
own volume number in its address field, all sectors on a disk usually have the same
one: Virtu gives each sector the volume number for the disk side it resides on (see
the Virtu "!Help"). DOS usually ignores volume numbers, but some software assigns
unique volume numbers to various disks in order to detect which one is in the drive.
In this case, set the volume number for each disk to that found by cataloging the
original disk or using a sector/nibble editor.

    Disk emulation is performed at the memory-mapped I/O level, not (as in some
emulators) by interception of RWTS (DOS) or MLI (ProDOS) calls. This technique
provides more complete emulation of disk access, though with a performance penalty.
Tracks are 'nibblized' at runtime in a manner analagous to DOS's method of writing
to disk. The resulting nibble data is almost identical to the data stream from a
disk ][ drive and interface card, save for a few changes made to fool the disk up-to-
speed check in RWTS (some emulators patch RWTS instead).

    The nibblizing of a track occurs only when the first nibble is actually
requested, not during disk arm movement. Non-integral track positions are truncated,
so track $11.5 reads track $11, and the maximum track is $22. A track is marked for
flushing when written to, and flushing occurs on:

- reading/writing a new track
- disk flip (A <-> B)
- changing drives (1 <-> 2)
- reset (also sets drive 1, drive motor off, stepper phases off)
- exiting Virtu

    The flush denibblizes the cached track image as DOS does when reading from disk
(except that Virtu ignores sector volume numbers), and if all sectors are error free
writes it back to the disk image. Initializing a disk should work for all DOSs that
use standard sector marks. Problems may occur in the unlikely event of a DOS
requiring unusual track lengths, inter-sector gap lengths or physical sector
orderings. These cases would, however, amount to copy protection anyway.

Video Emulation
~~~~~~~~~~~~~~~
    All standard Apple //e video modes are supported, including text, graphics and
mixed modes. When displaying hires graphics in color Virtu closely emulates the
characteristic odd/even pixel coloring which effectively halves the horizontal
screen resolution from 280 (monochrome) pixels to 140 (colour pair) pixels across.
The bit 7 (delay) half pixel shift and color interference of adjacent delayed and
non-delayed pixels is not emulated due to resolution and performance constraints.
The video modes supported are:

- Lores graphics
- 40 column Text
- 80 column Text
- Lores graphics & 40 column Text
- Lores graphics & 80 column Text
- Hires graphics
- Hires graphics & 40 column Text
- Hires graphics & 80 column Text
- 7M Lores graphics
- Double Lores graphics
- 7M Lores graphics & 40 column Text
- Double Lores graphics & 80 column Text
- ND Hires graphics
- Double Hires graphics
- ND Hires graphics & 40 column Text
- Double Hires graphics & 80 column Text

    7M Lores and ND Hires are little known, and hence little used, video modes which
offer standard modes with reduced color capability. 7M Lores results from viewing
normal Lores then accessing $C05E (DHiresSet). When 80 column mode is not enabled,
this forces normal Hires graphics timing (7Mhz) rather than double Hires timing
(14MHz). The reducton in timing resolution means only black, green, purple and white
colours are available. ND (no delay) Hires can be similarly activated, and because
$C05E disables the colour signal timing delay responsible for 'high bit set' colours
(orange and blue) when not in 80 column mode, the colours available are also only
those in 7M Lores. It seems that the only use for these modes would be to flash on
and off those colours affected for primitive special effects.

Joystick Emulation
~~~~~~~~~~~~~~~~~~
    A memory access in the range $C070 to $C07F starts timers whose pulse widths are
variable from 2 to 3300 microseconds depending on the resistances of the attached
joystick/paddles. The high bit of a paddle address goes low when the timer expires,
and most paddle routines count 11 cycle loops waiting for this change. Some software
may use longer loops to check multiple paddles, and hence the number of accesses is
not a reliable way to emulate these timers. Virtu keeps the signal high for a cycle
count equalling 11 times the paddle value (0 to 255).

Printer Emulation
~~~~~~~~~~~~~~~~~
    Only simple text printer output is supported. Memory writes in the range $C090
to $C09F are output to the printer output file after being ANDed with #$7F to strip
the high bit. Formatting control characters are not filtered out.

00000000  54 65 63 68 6e 69 63 61  6c 20 4e 6f 74 65 73 0a  |Technical Notes.|
00000010  3d 3d 3d 3d 3d 3d 3d 3d  3d 3d 3d 3d 3d 3d 3d 0a  |===============.|
00000020  0a 41 70 70 6c 65 32 65  52 4f 4d 20 49 6d 61 67  |.Apple2eROM Imag|
00000030  65 0a 7e 7e 7e 7e 7e 7e  7e 7e 7e 7e 7e 7e 7e 7e  |e.~~~~~~~~~~~~~~|
00000040  7e 7e 0a 20 20 20 20 54  68 65 20 41 70 70 6c 65  |~~.    The Apple|
00000050  32 65 52 4f 4d 20 69 6d  61 67 65 20 66 69 6c 65  |2eROM image file|
00000060  20 73 68 6f 75 6c 64 20  62 65 20 74 68 65 20 73  | should be the s|
00000070  61 6d 65 20 66 6f 72 6d  61 74 20 61 73 20 75 73  |ame format as us|
00000080  65 64 20 62 79 20 74 68  65 20 41 70 70 6c 65 57  |ed by the AppleW|
00000090  69 6e 0a 65 6d 75 6c 61  74 6f 72 20 28 66 6f 72  |in.emulator (for|
000000a0  20 74 68 65 20 49 42 4d  20 50 43 29 20 61 6e 64  | the IBM PC) and|
000000b0  20 69 73 20 61 73 20 66  6f 6c 6c 6f 77 73 3a 0a  | is as follows:.|
000000c0  0a 2b 26 30 30 30 30 20  45 78 74 65 72 6e 61 6c  |.+&0000 External|
000000d0  20 49 2f 4f 20 52 4f 4d  20 28 24 43 30 30 30 2d  | I/O ROM ($C000-|
000000e0  24 43 46 46 46 29 0a 2b  26 31 30 30 30 20 49 6e  |$CFFF).+&1000 In|
000000f0  74 65 72 6e 61 6c 20 49  2f 4f 20 52 4f 4d 20 28  |ternal I/O ROM (|
00000100  24 43 30 30 30 2d 24 43  46 46 46 29 0a 2b 26 32  |$C000-$CFFF).+&2|
00000110  30 30 30 20 4d 6f 6e 69  74 6f 72 20 52 4f 4d 20  |000 Monitor ROM |
00000120  20 20 20 20 20 28 24 44  30 30 30 2d 24 46 46 46  |     ($D000-$FFF|
00000130  46 29 0a 0a 50 72 6f 63  65 73 73 6f 72 20 45 6d  |F)..Processor Em|
00000140  75 6c 61 74 69 6f 6e 0a  7e 7e 7e 7e 7e 7e 7e 7e  |ulation.~~~~~~~~|
00000150  7e 7e 7e 7e 7e 7e 7e 7e  7e 7e 7e 0a 20 20 20 20  |~~~~~~~~~~~.    |
00000160  54 68 65 20 4e 4d 4f 53  20 36 35 30 32 20 68 61  |The NMOS 6502 ha|
00000170  64 20 73 65 76 65 72 61  6c 20 70 72 6f 62 6c 65  |d several proble|
00000180  6d 73 20 77 68 69 63 68  20 77 65 72 65 20 72 65  |ms which were re|
00000190  63 74 69 66 69 65 64 20  69 6e 20 6c 61 74 65 72  |ctified in later|
000001a0  20 6d 6f 64 65 6c 73 20  69 6e 20 74 68 65 0a 73  | models in the.s|
000001b0  65 72 69 65 73 2e 20 56  69 72 74 75 20 61 63 63  |eries. Virtu acc|
000001c0  75 72 61 74 65 6c 79 20  65 6d 75 6c 61 74 65 73  |urately emulates|
000001d0  20 74 68 65 20 6b 6e 6f  77 6e 20 62 75 67 73 20  | the known bugs |
000001e0  66 6f 75 6e 64 20 69 6e  20 74 68 65 20 6f 72 69  |found in the ori|
000001f0  67 69 6e 61 6c 3a 0a 0a  2d 20 6a 75 6d 70 20 69  |ginal:..- jump i|
00000200  6e 64 69 72 65 63 74 20  77 72 61 70 73 20 74 68  |ndirect wraps th|
00000210  65 20 65 66 66 65 63 74  69 76 65 20 61 64 64 72  |e effective addr|
00000220  65 73 73 20 61 74 20 61  20 70 61 67 65 20 62 6f  |ess at a page bo|
00000230  75 6e 64 61 72 79 0a 2d  20 69 6e 76 61 6c 69 64  |undary.- invalid|
00000240  20 66 6c 61 67 73 20 61  66 74 65 72 20 61 6e 20  | flags after an |
00000250  61 72 69 74 68 6d 65 74  69 63 20 6f 70 65 72 61  |arithmetic opera|
00000260  74 69 6f 6e 20 69 6e 20  64 65 63 69 6d 61 6c 20  |tion in decimal |
00000270  6d 6f 64 65 0a 0a 20 20  20 20 41 6e 6f 74 68 65  |mode..    Anothe|
00000280  72 20 70 72 6f 62 6c 65  6d 20 77 69 74 68 20 74  |r problem with t|
00000290  68 65 20 4e 4d 4f 53 20  36 35 30 32 20 77 61 73  |he NMOS 6502 was|
000002a0  20 74 68 65 20 6f 70 65  72 61 74 69 6f 6e 20 6f  | the operation o|
000002b0  66 20 75 6e 64 65 66 69  6e 65 64 20 6f 72 0a 27  |f undefined or.'|
000002c0  75 6e 64 6f 63 75 6d 65  6e 74 65 64 27 20 6f 70  |undocumented' op|
000002d0  63 6f 64 65 73 20 77 68  69 63 68 20 77 65 72 65  |codes which were|
000002e0  20 65 78 65 63 75 74 61  62 6c 65 2c 20 62 75 74  | executable, but|
000002f0  20 6e 6f 74 20 6c 65 67  61 6c 20 69 6e 73 74 72  | not legal instr|
00000300  75 63 74 69 6f 6e 73 2e  20 4d 6f 73 74 20 6f 66  |uctions. Most of|
00000310  0a 74 68 65 73 65 20 6f  70 63 6f 64 65 73 20 72  |.these opcodes r|
00000320  65 73 75 6c 74 20 69 6e  20 74 68 65 20 43 50 55  |esult in the CPU|
00000330  20 61 74 74 65 6d 70 74  69 6e 67 20 74 6f 20 65  | attempting to e|
00000340  66 66 65 63 74 69 76 65  6c 79 20 65 78 65 63 75  |ffectively execu|
00000350  74 65 20 74 77 6f 20 6c  65 67 61 6c 0a 69 6e 73  |te two legal.ins|
00000360  74 72 75 63 74 69 6f 6e  73 20 61 74 20 6f 6e 63  |tructions at onc|
00000370  65 2c 20 61 6e 64 20 74  68 75 73 20 70 72 6f 64  |e, and thus prod|
00000380  75 63 65 20 6f 62 73 63  75 72 65 2c 20 61 6e 64  |uce obscure, and|
00000390  20 65 76 65 6e 20 61 6d  75 73 69 6e 67 20 6f 70  | even amusing op|
000003a0  65 72 61 74 69 6f 6e 73  2e 20 56 69 72 74 75 0a  |erations. Virtu.|
000003b0  69 6d 70 6c 65 6d 65 6e  74 73 20 74 68 65 73 65  |implements these|
000003c0  20 6f 70 63 6f 64 65 73  20 61 73 20 6d 75 6c 74  | opcodes as mult|
000003d0  69 2d 62 79 74 65 20 4e  4f 50 20 69 6e 73 74 72  |i-byte NOP instr|
000003e0  75 63 74 69 6f 6e 73 20  61 73 20 70 65 72 20 74  |uctions as per t|
000003f0  68 65 20 6f 70 65 72 61  6e 64 20 6c 65 6e 67 74  |he operand lengt|
00000400  68 20 6f 66 0a 74 68 65  20 6f 70 63 6f 64 65 73  |h of.the opcodes|
00000410  20 69 6e 20 61 20 52 6f  63 6b 77 65 6c 6c 20 52  | in a Rockwell R|
00000420  36 35 30 32 2e 20 54 68  65 20 63 79 63 6c 65 20  |6502. The cycle |
00000430  74 69 6d 65 20 75 73 65  64 20 69 73 20 74 68 65  |time used is the|
00000440  20 6d 61 78 69 6d 75 6d  20 6f 66 20 74 68 65 20  | maximum of the |
00000450  74 69 6d 65 73 20 66 6f  72 0a 65 61 63 68 20 6f  |times for.each o|
00000460  66 20 74 68 65 20 74 77  6f 20 6c 65 67 61 6c 20  |f the two legal |
00000470  69 6e 73 74 72 75 63 74  69 6f 6e 73 20 74 68 61  |instructions tha|
00000480  74 20 63 6f 6d 70 72 69  73 65 20 74 68 65 20 75  |t comprise the u|
00000490  6e 64 65 66 69 6e 65 64  20 69 6e 73 74 72 75 63  |ndefined instruc|
000004a0  74 69 6f 6e 2e 0a 0a 20  20 20 20 54 68 65 20 36  |tion...    The 6|
000004b0  35 43 30 32 20 61 6d 65  6e 64 73 20 61 6c 6c 20  |5C02 amends all |
000004c0  74 68 65 20 70 72 6f 62  6c 65 6d 73 20 6d 65 6e  |the problems men|
000004d0  74 69 6f 6e 65 64 20 61  62 6f 76 65 3b 20 68 6f  |tioned above; ho|
000004e0  77 65 76 65 72 20 73 6f  6d 65 20 6f 6c 64 65 72  |wever some older|
000004f0  20 73 6f 66 74 77 61 72  65 0a 6d 61 79 20 72 65  | software.may re|
00000500  6c 79 20 6f 6e 20 74 68  69 73 20 65 72 72 6f 6e  |ly on this erron|
00000510  65 6f 75 73 20 66 75 6e  63 74 69 6f 6e 61 6c 69  |eous functionali|
00000520  74 79 2c 20 74 68 75 73  20 56 69 72 74 75 27 73  |ty, thus Virtu's|
00000530  20 63 61 70 61 62 69 6c  69 74 79 20 74 6f 20 73  | capability to s|
00000540  77 69 74 63 68 20 62 65  74 77 65 65 6e 0a 74 68  |witch between.th|
00000550  65 20 74 77 6f 20 70 72  6f 63 65 73 73 6f 72 73  |e two processors|
00000560  20 62 65 63 6f 6d 65 73  20 64 65 73 69 72 61 62  | becomes desirab|
00000570  6c 65 2e 20 49 6e 20 70  61 72 74 69 63 75 6c 61  |le. In particula|
00000580  72 2c 20 74 68 65 20 69  6e 64 69 72 65 63 74 20  |r, the indirect |
00000590  6a 75 6d 70 20 61 6e 64  20 75 6e 64 65 66 69 6e  |jump and undefin|
000005a0  65 64 0a 69 6e 73 74 72  75 63 74 69 6f 6e 73 20  |ed.instructions |
000005b0  77 65 72 65 20 73 6f 6d  65 74 69 6d 65 73 20 75  |were sometimes u|
000005c0  73 65 64 20 74 6f 20 6d  61 6b 65 20 63 6f 70 79  |sed to make copy|
000005d0  20 70 72 6f 74 65 63 74  69 6f 6e 20 72 6f 75 74  | protection rout|
000005e0  69 6e 65 73 20 68 61 72  64 20 74 6f 20 66 6f 6c  |ines hard to fol|
000005f0  6c 6f 77 2c 0a 61 6e 64  20 73 6f 6d 65 20 67 61  |low,.and some ga|
00000600  6d 65 73 20 6d 61 79 20  72 65 6c 79 20 6f 6e 20  |mes may rely on |
00000610  74 68 65 20 69 6e 63 6f  72 72 65 63 74 20 64 65  |the incorrect de|
00000620  63 69 6d 61 6c 20 61 72  69 74 68 6d 65 74 69 63  |cimal arithmetic|
00000630  20 72 65 73 75 6c 74 20  66 6c 61 67 73 20 66 6f  | result flags fo|
00000640  72 20 73 63 6f 72 65 2d  0a 6b 65 65 70 69 6e 67  |r score-.keeping|
00000650  2e 20 4e 6f 74 65 20 61  6c 73 6f 20 74 68 61 74  |. Note also that|
00000660  20 61 6c 74 68 6f 75 67  68 20 74 68 65 20 36 35  | although the 65|
00000670  43 30 32 20 64 65 66 69  6e 65 73 20 74 68 65 20  |C02 defines the |
00000680  36 35 30 32 27 73 20 75  6e 64 65 66 69 6e 65 64  |6502's undefined|
00000690  20 6f 70 63 6f 64 65 73  20 61 73 0a 65 69 74 68  | opcodes as.eith|
000006a0  65 72 20 6e 65 77 20 69  6e 73 74 72 75 63 74 69  |er new instructi|
000006b0  6f 6e 73 20 6f 72 20 6d  75 6c 74 69 2d 62 79 74  |ons or multi-byt|
000006c0  65 20 4e 4f 50 73 2c 20  74 68 65 20 6e 75 6d 62  |e NOPs, the numb|
000006d0  65 72 20 6f 66 20 62 79  74 65 73 20 63 6f 6e 73  |er of bytes cons|
000006e0  75 6d 65 64 20 28 69 65  20 6f 70 65 72 61 6e 64  |umed (ie operand|
000006f0  0a 6c 65 6e 67 74 68 29  20 64 69 66 66 65 72 73  |.length) differs|
00000700  2e 0a 0a 53 6f 75 6e 64  20 45 6d 75 6c 61 74 69  |...Sound Emulati|
00000710  6f 6e 0a 7e 7e 7e 7e 7e  7e 7e 7e 7e 7e 7e 7e 7e  |on.~~~~~~~~~~~~~|
00000720  7e 7e 0a 20 20 20 20 4d  65 6d 6f 72 79 20 61 63  |~~.    Memory ac|
00000730  63 65 73 73 65 73 20 69  6e 20 74 68 65 20 72 61  |cesses in the ra|
00000740  6e 67 65 20 24 43 30 33  30 20 74 6f 20 24 43 30  |nge $C030 to $C0|
00000750  33 46 2c 20 62 6f 74 68  20 72 65 61 64 73 20 61  |3F, both reads a|
00000760  6e 64 20 77 72 69 74 65  73 2c 20 67 65 6e 65 72  |nd writes, gener|
00000770  61 74 65 20 61 0a 73 70  65 61 6b 65 72 20 63 6c  |ate a.speaker cl|
00000780  69 63 6b 20 6f 6e 20 61  6e 20 41 70 70 6c 65 20  |ick on an Apple |
00000790  5d 5b 2e 20 56 69 72 74  75 20 62 75 66 66 65 72  |][. Virtu buffer|
000007a0  73 20 73 75 63 68 20 61  63 63 65 73 73 65 73 20  |s such accesses |
000007b0  61 6e 64 20 74 68 65 20  56 56 6f 69 63 65 20 6d  |and the VVoice m|
000007c0  6f 64 75 6c 65 20 75 73  65 73 0a 74 68 69 73 20  |odule uses.this |
000007d0  69 6e 66 6f 72 6d 61 74  69 6f 6e 20 74 6f 20 67  |information to g|
000007e0  65 6e 65 72 61 74 65 20  73 71 75 61 72 65 2d 77  |enerate square-w|
000007f0  61 76 65 20 70 75 6c 73  65 73 20 28 73 63 61 6c  |ave pulses (scal|
00000800  65 64 20 62 79 20 76 6f  6c 75 6d 65 29 20 66 6f  |ed by volume) fo|
00000810  72 20 74 68 65 20 73 6f  75 6e 64 0a 73 79 73 74  |r the sound.syst|
00000820  65 6d 2e 20 49 6e 74 65  6e 73 65 20 73 70 65 61  |em. Intense spea|
00000830  6b 65 72 20 61 63 74 69  76 69 74 79 20 61 74 20  |ker activity at |
00000840  61 63 63 65 73 73 20 70  65 72 69 6f 64 73 20 6f  |access periods o|
00000850  66 20 61 72 6f 75 6e 64  20 34 38 20 6d 69 63 72  |f around 48 micr|
00000860  6f 73 65 63 6f 6e 64 73  20 6f 72 20 6c 65 73 73  |oseconds or less|
00000870  0a 77 6f 75 6c 64 20 27  62 6f 67 20 64 6f 77 6e  |.would 'bog down|
00000880  27 20 74 68 65 20 73 6f  75 6e 64 20 65 6d 75 6c  |' the sound emul|
00000890  61 74 69 6f 6e 2c 20 73  69 6e 63 65 20 74 68 69  |ation, since thi|
000008a0  73 20 69 73 20 74 68 65  20 6f 75 74 70 75 74 20  |s is the output |
000008b0  73 61 6d 70 6c 65 20 70  65 72 69 6f 64 20 66 6f  |sample period fo|
000008c0  72 0a 74 68 65 20 73 69  6e 67 6c 65 20 73 6f 75  |r.the single sou|
000008d0  6e 64 20 63 68 61 6e 6e  65 6c 20 75 73 65 64 2e  |nd channel used.|
000008e0  20 54 6f 20 70 72 65 76  65 6e 74 20 74 68 69 73  | To prevent this|
000008f0  2c 20 61 63 63 65 73 73  65 73 20 61 72 65 20 69  |, accesses are i|
00000900  67 6e 6f 72 65 64 20 69  66 20 74 68 65 79 20 6f  |gnored if they o|
00000910  63 63 75 72 0a 61 74 20  69 6e 74 65 72 76 61 6c  |ccur.at interval|
00000920  73 20 6f 66 20 61 72 6f  75 6e 64 20 34 38 20 6d  |s of around 48 m|
00000930  69 63 72 6f 73 65 63 6f  6e 64 73 20 6f 72 20 6c  |icroseconds or l|
00000940  65 73 73 2e 20 41 6c 74  68 6f 75 67 68 20 61 20  |ess. Although a |
00000950  73 6d 61 6c 6c 65 72 20  6f 75 74 70 75 74 20 70  |smaller output p|
00000960  65 72 69 6f 64 0a 77 6f  75 6c 64 20 69 6d 70 72  |eriod.would impr|
00000970  6f 76 65 20 73 6f 75 6e  64 20 72 65 73 6f 6c 75  |ove sound resolu|
00000980  74 69 6f 6e 2c 20 69 74  20 73 69 67 6e 69 66 69  |tion, it signifi|
00000990  63 61 6e 74 6c 79 20 69  6e 63 72 65 61 73 65 73  |cantly increases|
000009a0  20 6f 76 65 72 68 65 61  64 73 3b 20 34 38 75 73  | overheads; 48us|
000009b0  20 69 73 20 61 0a 63 6f  6d 70 72 6f 6d 69 73 65  | is a.compromise|
000009c0  20 74 68 61 74 20 67 69  76 65 73 20 61 20 66 75  | that gives a fu|
000009d0  6c 6c 65 72 20 73 6f 75  6e 64 20 66 6f 72 20 74  |ller sound for t|
000009e0  68 65 20 61 76 65 72 61  67 65 20 63 61 73 65 2e  |he average case.|
000009f0  0a 0a 44 69 73 6b 20 5d  5b 20 44 72 69 76 65 20  |..Disk ][ Drive |
00000a00  45 6d 75 6c 61 74 69 6f  6e 0a 7e 7e 7e 7e 7e 7e  |Emulation.~~~~~~|
00000a10  7e 7e 7e 7e 7e 7e 7e 7e  7e 7e 7e 7e 7e 7e 7e 7e  |~~~~~~~~~~~~~~~~|
00000a20  7e 0a 20 20 20 20 56 69  72 74 75 20 63 75 72 72  |~.    Virtu curr|
00000a30  65 6e 74 6c 79 20 73 75  70 70 6f 72 74 73 20 6f  |ently supports o|
00000a40  6e 6c 79 20 6f 6e 65 20  74 79 70 65 20 6f 66 20  |nly one type of |
00000a50  41 70 70 6c 65 20 5d 5b  20 64 69 73 6b 20 69 6d  |Apple ][ disk im|
00000a60  61 67 65 2c 20 74 68 65  20 44 53 4b 20 28 66 72  |age, the DSK (fr|
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00001ec0  31 31 20 74 69 6d 65 73  20 74 68 65 20 70 61 64  |11 times the pad|
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00001ee0  32 35 35 29 2e 0a 0a 50  72 69 6e 74 65 72 20 45  |255)...Printer E|
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00001f30  73 20 73 75 70 70 6f 72  74 65 64 2e 20 4d 65 6d  |s supported. Mem|
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00001f60  20 24 43 30 39 46 20 61  72 65 20 6f 75 74 70 75  | $C09F are outpu|
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00001fc0  46 6f 72 6d 61 74 74 69  6e 67 20 63 6f 6e 74 72  |Formatting contr|
00001fd0  6f 6c 20 63 68 61 72 61  63 74 65 72 73 20 61 72  |ol characters ar|
00001fe0  65 20 6e 6f 74 20 66 69  6c 74 65 72 65 64 20 6f  |e not filtered o|
00001ff0  75 74 2e 0a                                       |ut..|
00001ff4

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