Commodore 64: Visiload loader
This is a VERY common loader, especially for Codemasters games, rather annoyingly though it does NOT contain checksums.
Can be found on... 1985, Action Biker, Advanced Pinball Simulator, After the War, ATV Simulator, American 3D Pool, Arcade Flight Simulator, Big Mac, Bigfoot, BMX Freestyle, Bump-Set-Spike, Cage Match, Carnage, Draconus, ALL the "Dizzy" Games, Human Race, Kikstart, Pod, etc..
There appears to be at least 4 types of this loader, each loader is basically identical but they each set different thresholds...
- Type 1 (threshold : $1B6 clock cycles, TAP byte $37)
- Type 2 (threshold : $1E6 clock cycles, TAP byte $3D)
- Type 3 (threshold : $1F8 clock cycles, TAP byte $3F)
- Type 4 (threshold : $243 clock cycles, TAP byte $48)
Bit 1's sampled from TAP files show values of $48+ for type 1, whilst sampled bit 1's from type 2 TAPs are typically around $4B+.
Type 3's bit 1's are found as high as $5B. Type 4's bit 1's are found as high as $64.
Bit 0's for types 1-3 seem roughly similar, being around $25.
Bit 0's for type 4 are around $30.
--------------------------------------------------------------------------------
Type1 Type2 Type3 Type4
Bit 0 : $25 $25 $25 $30 (sampled)
Bit 1 : $48 $4B $54 $5D (sampled)
Thres : $35 $3B $3E $47 (exact)
Endianess : It changes during the loading! as does the number of bits per byte!
Pilot byte : $00 (*256)
Sync byte : $16
Some blocks have the Pilot tone, some don't. First block has 9 bits per byte (additional bit is sent before the actual byte and it always has value 1), it has MSbF endianess and it has 256 bytes of Pilot tone with value $00.
eg..
Pilot 1:0000:0000 ($00) *256
sync 1:0001:0110 ($16) Structure:
This loader changes its structure DURING actual loading (by loading 'modifier' blocks over itself). The blocks that are loaded in are of different length, different endianess, have different number of additional bits before each byte (their value is always 1) and have different number of header bytes. Also sometimes they have pilot bytes and sometimes they don't.
You can determine all this by looking at the starting address of the block (see bottom of page).
Initial values of loading are :
Data Endianess : MSbF
Pilot tone : yes
Additional bits per byte : 1
Additional header bytes : 0
If you should read some additional bits you must read them before actual byte starts, this is done for all bytes (including Pilot bytes). So the starting pilot byte that should be read in is actually 1:0000:0000 (additional bit is 1 and the byte has value $00).
Block structure :
If pilot should be read then there are any number of bytes $00 followed by the sync byte $16.
If the pilot should not be read then there is no sync byte either.
After that the additional Header bytes are sent (default is 0, but can be changed later!)
Then the following 4 bytes are sent :
- 2 bytes : Block END address HIGH, LOW
- 2 bytes : Block START address HIGH, LOW
After that the actual DATA is sent
NOTE: if Block END == Block START then 1 byte will STILL be sent!
AFTER you read in all the data you must look at the START address, here are the special cases that you must consider :-
- $034b : First DATA byte holds number of bits per byte for next block(s)
- $03a4 : First DATA byte holds number of additional header bytes + 3
- $0347 : if First DATA byte is $26 then next block will be MSbF else LSbF
- $03bb : Next block will have PILOT tone before it (ANY number , can be 1 !)
These values change after every 2 or 3 blocks, so the structure changes constantly throughout the loading.
Notes:
At the end of a visiload chain is often a few stray pulses (5 or more), these can be safely removed if desired. they occur typically at the end of a chain immediately before a pause.
