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|| .PCV (PCVIC SYSTEM SNAPSHOT) FORMAT SPECIFICATIONS ||
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|| Version 1.00 ||
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|| as generated & used by PCVIC 1.0 - 1.14 ||
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|| (C) Copyright 16 jan 1998 by B.W. van Schooten ||
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These specifications should include this file PCVHEAD.DOC and the C source
PCVDUMP.C which decodes a PCV image and provides some diagnostic information.
Care has been taken to make sure these specifications are clear and correct,
though the author cannot guarantee that they are. If you find any bugs or
inconsistencies, please report them to me (my current email address is
vicman@dds.nl). If you are interested in additions to the snapshot format,
please report them to me, so we can make sure the format stays consistent.
The PCVIC system snapshot (.PCV)
A system snapshot stores the complete state of the emulated Vic-20, and can be
used to save and restore any program at any point within its execution.
Offset
$ 0 --- 'PCVIC system snapshot',0
$16 --- Version number (minor, major)
$18 --- Size of register state block
$1a --- Register state block: System register values and memory expansion
setting.
mmm --- Byterun compressed image of the Vic's memory.
The areas where the system ROMs are, $8000..$8FFF and $C000..$FFFF,
are excluded.
nnn --- Two-byte checksum of the data starting from offset $16 but excluding
the checksum itself
All words described here are little-endian (low byte first) encoded.
-----------------------------------------
$16 --- Version number (minor, major)
-----------------------------------------
0 (BYTE) MINORVERSION
Value between 0 .. 99.
Examples: V1.12 -> minor version=12, V1.01 -> minor version=1
1 (BYTE) MAJORVERSION
Value between 0 .. 255.
----------------------------------------
$18 --- Size of register state block
----------------------------------------
0 (WORD) RSBSIZE
Size of the register state block, not including the size field itself.
-----------------------------------------------------------------------------
$1A --- Register state block: System register values and memory expansion
setting.
-----------------------------------------------------------------------------
0 (WORD) X
Low byte = 6502 X register
High byte must be set to 0
2 (WORD) Y
Low byte = 6502 Y register
High byte must be set to 0
4 (WORD) S
Low byte = 6502 S register
High byte must be set to 1
6 (BYTE) OLDSCANCOUNT
Scrap register; don't care
7 (BYTE) AUXFLAGS
Little-used 6502 flags:
Bit2..Bit5 are Bit2..Bit5 (1,B,D,I) of P register.
The other bits should be set to 0.
8 (WORD) SCANLINE
ScanLine nr. 0..263(NTSC) or 0..311(PAL)
Values up to 32767 are possible.
10 (4 BYTES) IFRIER
Exact copies of the IFR and IER registers.
(Order is same as in the vic's memory)
10 (BYTE) IFR1 VIA1: IFR register
11 (BYTE) IER1 VIA1: IER register
12 (BYTE) IFR2 VIA2: IFR register
13 (BYTE) IER2 VIA2: IER register
14 (8 BYTES) PORTDATA
Values as found in the IRA/ORA & IRB/ORB latches.
Order is the same as in memory.
var.name chip reg.name
14 (BYTE) PERIP1B VIA1 IRB
15 (BYTE) LATCH1B VIA1 ORB
16 (BYTE) PERIP1A VIA1 IRA
17 (BYTE) LATCH1A VIA1 ORA
18 (BYTE) PERIP2B VIA2 IRB
19 (BYTE) LATCH2B VIA2 ORB
20 (BYTE) PERIP2A VIA2 IRA
21 (BYTE) LATCH2A VIA2 ORA
22 (BYTE) TIMERSTATUS
Lower 4 bits correspond to status of VIA timers:
bit0 1 = VIA1 Timer1 running
bit1 1 = VIA1 Timer2 running
bit2 1 = VIA2 Timer1 running
bit3 1 = VIA2 Timer2 running
High 4 bits are future expansion: set these to 0.
23 (4 BYTES) TIMERDATA
A copy of the low bytes of the VIAs' latches and timers. These must be
kept consistent with the values found in the memory area $9110-$912F.
23 (BYTE) V1T2LATCHL
24 (BYTE) V2T2LATCHL
25 (BYTE) V1T2TIMERL
26 (BYTE) V2T2TIMERL
27 (BYTE) NMIFLANK
This variable is used to detect NMI flanks.
0 = NMI was low since last time the NMI pin was sampled.
Other = NMI high since last time sampled.
28 (BYTE) MEMCONFIG
Each bit corresponds to the presence of an 8k block.
Bit=1 => area is RAM
Bit=0 => area is write-protected
(for ROM areas or unused addressing space)
Bit: 7 6 5 4 3 2 1 0
8K-Block start: xxxx xxxx A000 xxxx 6000 4000 2000 0000
xxxx means: Unavailable. (the system ROMs and I/O space are situated
here) Always set these bits to 0.
Note that the areas 0000..03ff and 1000..2000 are considered to be
always occupied by RAM. For an unexpanded Vic, all blocks are set to
0. Setting only the low block (bit0) it to 1 results in 3K RAM
expansion.
29 (WORD) PC
6502 PC register
31 (WORD) MAINFLAGS
Much-used 6502 flags.
Note that: '.' means: 'don't care'.
'0' means: always set to 0.
Bit: 7654 3210
Low byte = SZ.. ....
High byte = 0V00 000C
33 (BYTE) A
6502 A register
34 (BYTE) SCANCOUNT
CPU cycle count within scan line.
0 = End of scan line
129(NTSC) or 131(PAL) = Start of scan line
35 END
-------------------------------------------------------------------------------
mmm --- Byterun compressed image of the Vic's memory.
The areas where the system ROMs are, $8000..$8FFF and $C000..$FFFF,
are excluded.
-------------------------------------------------------------------------------
Byterun (as is also used in ILBM images) achieves compression by replacing a
row of bytes with equal values by a count and a value byte.
See the procedure 'decompress' in the C source 'PCVDUMP.C' for a detailed
description of how to decompress byterun-encoded data.
There are two noncontiguous memory blocks in the image, $0000..$7FFF and
$9000..$BFFF, which are compressed separately.
$0000..$7FFF Memory data
This complete area is always included, regardless of the value of
MEMCONFIG; some of it could be occupied by ROMs.
$9000..$93FF I/O Space
Only the areas $9000..$900F and $9110..$912F are defined. These
should contain the values as found when reading from the corresponding
VIC and VIA registers.
The port addresses on a real Vic are shadowed multiple times across
this area, though current versions of PCVIC do not support this. This
means PCVIC will fill the rest of the area with '$FF' bytes. The values
found in these shadow areas are 'don't care'. The remaining values are
undefined.
$9400..$97FF Colour memory
Only the low nybbles are defined.
$9800..$9FFF Free I/O Space
In PCV version 1.0, there is no room in the register state block for
optional special hardware which could have their I/O space here.
However, it is possible to map ROM to this area, and probably RAM as
well.
$A000..$CFFF Memory data
This complete area is always included, regardless of the value of
MEMCONFIG; some of it could be occupied by ROMs.
------------------------------------------------------------------------------
nnn --- Two-byte checksum of the data starting from offset $16 but
(of course) excluding the checksum itself
------------------------------------------------------------------------------
The checksum in initialised with value $7ffb. The checksum is then updated with
each byte read from the PCV file. See the procedure 'addtochecksum' in the C
source 'PCVDUMP.C' for the checksum update algorithm.
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