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Copy protection: David23

[BBC Micro cassette version, Loader]

Name: David23 [Show more] Type: Variable Category: Copy protection Summary: Address pointer to the start of the 6502 stack
Context: See this variable in context in the source code Variations: See code variations for this variable in the different versions References: This variable is used as follows: * Elite loader (Part 4 of 6) uses David23

This two-byte address points to the start of the 6502 stack, which descends from the end of page 2, less LEN bytes, which comes out as &01DF. So when we push 33 bytes onto the stack (LEN being 33), this address will point to the start of those bytes, which means we can push executable code onto the stack and run it by calling this address with a JMP (David23) instruction. Sneaky stuff!
.David23 EQUW (512-LEN) \ The address of LEN bytes before the start of the stack