Commodore 64: Super Pavloda loader

Published in

· 3 years ago

Frequently used by 'Melbourne House' games, it can be found on...
Way of the Exploding Fist, Fist 2, Bazooka Bill, Gyroscope, Knucklebusters...

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Endianess :
MSbF

Uses 3 pulse types, Short, Medium and Long.

2 "pulse-sets" have been found for this loader so far...

Super Pavloda T1 (used on "Way Of The Exploding Fist")

Short : $2E
Medium : $45
Long : $5C

Super Pavloda T2 (used on "Fist 2")

Short : $3E
Medium : $5D
Long : $7C

Note :
a tape will only use one set, they are never mixed.

Encoding...

Pulses have a different meaning, depending on the current "Status"...

When in Status 1...

 -------------------- 
 Pulse  | Bit pattern  
 -------------------- 
 Short  | 1  
 Medium | 00 
 Long   | 01 
 --------------------

When in Status 2...

 -------------------- 
 Pulse  | Bit pattern  
 -------------------- 
 Short  | 0  
 Medium | 1  
 Long   | 00 
 --------------------

Status may be modified at each pulse reading...

 -------------------- 
 Pulse  | Status 
 -------------------- 
 Short  | Keep current 
 Medium | Switch to the other 
 Long   | Keep current 
 --------------------

Initial status = 2.

Structure:
Bits are lined from left to right. In other words: MSbF.

AFTER syncronization has been done , you may group bits (by eight) to form valid data bytes.
Since bit encoding was used, it may happen last bit of a byte and first bit of next byte were coded together (eg: .......0 0....... or .......0 1.......).

Most TAP dumps examined seem to have some crap before and after the pauses between blocks, usually this is 4,5 or 6 pulses at most and tests have shown it is safe to remove.

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Data is structured as a chain of one or more sub-blocks, there are two types of sub-block, Primary and Secondary, the first block in a chain is always Primary type and contains a more detailed header section...

PRIMARY sub-block...

Pilot :
Sequence of around 2068 Short pulses ending in a Medium pulse.
Sync bytes : $66,$1B

1 byte : Block Number (they begin at 0)
1 byte : Sub-block number (they begin at 0)
2 bytes : Load address (low,high) -Load offset.
1 byte : Total data size (high)
1 byte : Load offset (== 256-Total file size low byte) add to load address to get real one.
1 byte : Header Checkbyte (addition of previous 6 bytes +6)
n bytes : Data (n = Total data size & $FF)
1 byte : Checksum (0 xor all n data bytes)

SECONDARY sub-block (only present if total data size exceeds 256 bytes)...

Pilot :
Usually around 40 Short pulses followed by a Medium pulse.
Sync bytes : $66,$1B

1 byte : Block Number (the same for all blocks in any chain)
1 byte : Sub-Block Number
256 bytes : Data
1 byte : Checksum (0 XOR all 258 bytes, +2)

----------------------

The Primary sub-block provides the load address and file size of ALL data contained in all sub-blocks in the chain.

Data is distributed throughout the sub-blocks as follows...

If Total data size divides equally by 256, then each sub-block will contain 256 data bytes, otherwise the first (Primary) block will contain a number of bytes equal to the remainder.

ie. if total_data_size = 1027, the Primary sub-block will contain 3 data bytes. (1027 & 255).

Secondary sub-blocks (if present) will always contain 256 data bytes.