session-two: variables-example about hackaday-u HOT 4 CLOSED

Hey @xmpf,

you've correctly located the instruction that gives that output :

While the 8-bits registry CL is (always) used for SAR second variable operand, reading the Intel x64 specs for SAR gives :

(Intel Software Developer’s Manual, Volume 2B, page 4-584)

and the corresponding bits for AMD64 :

(AMD64 Architecture Programmer's Manual volume 3, page 318)

So when the compiler is translating the >> operator to a SAR instruction, it's implicit that the right operand will be used modulo 64 (or modulo 32 if using a 32-bits wide register).
Ghidra, being a friendly program, slaps an artificial & 0x3F in its decompilation to reminds the reverser that fact.

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You're welcome.
If you're refering to :

then this logical AND applies to the whole expression, not only index*8 and truncate the number to fit inside an unsigned char.

This is visible in the decompiled code as a (char) cast :

or in the disassembly when the 8-bits register AL is used to store the result :

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Thanks :)

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Hi, thank you for your answer.
In the original source code however, the modulo is 0xff.

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