Commodore 64: Action Replay cartridge loader

@DrWatson

Published in 

C64 tape formats

 · 31 May 2021

Action Replay cartridge loader
=================
info by Fabrizio Gennari <fabrizio.ge@tiscalinet.it>

Used by the Action Replay cartridge to save freezed games.

Encoding :

In the pilot tone, sync sequence and first bytes of header, pulses are like this:
Threshold : $1D0 (464) clock cycles
Bit 0 : TAP byte $23
Bit 1 : TAP byte $53

Data Endianess : LEAST Significant bit First

Lead-In : a stream of 1's, typically about 2048 of them, followed by a single 0 bit
Sync sequence: $52, $42

Structure:

 00-01 Complement of length of data (High, Low byte) 
 02-03 Start Address of data        (High, Low byte) 
 04    Checksum (XOR of all data bytes) 
 05    discarded (typically $00) 
 06    Low byte of threshold for data bytes 
 07    discarded (typically $00) 
 08    Data bytes 

To calculate the number of data bytes, do 65536-complement of length. To calculate threshold for data bytes, do $100+low byte. There are two possibilities for the threshold: $1D0 (in that case, following pulses are identical to previous ones: this is the case of Turbo backup) and $111 (in that case, pulses are TAP bytes $13 for 0 and $2B for 1: that is the case of Superturbo backup)