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Copy protection: PROT2

[BBC Micro disc version, Loader 3]

Name: PROT2 [Show more] Type: Subroutine Category: Copy protection Summary: Part of the CHKSM copy protection checksum calculation
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * PROT1 calls PROT2
.PROT2 LDA RAND+2 \ Fetch the checksum we calculated in PROT1 EOR CHKSM \ Set A = A EOR CHKSM ASL A \ Shift A left, moving bit 7 into the C flag and \ clearing bit 0 CMP #147 \ If A >= 147, set the C flag, otherwise clear it ROR A \ Shift A right, moving the C flag into bit 7 and \ clearing the C flag STA CHKSM \ Store the updated A in CHKSM BCC out \ Return from the subroutine (as we cleared the C flag \ above and out contains an RTS)