Trouble understanding an exercise given the two's complement in hex format to convert into decimal format

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I am trying to convert from hex values in two's complement to the decimal value of the following:

23, 57, 94 and 87.

a) 23

Procedure: (3 x 16^0) + (2 x 16^1) -> (3) + (32) = 35 (Correct)

b) 57

Procedure: (7 x 16^0) + (5 x 16^1) -> (7) + (80) = 87 (Correct)

For 94 and 87, the correct values are -108 & -121 respectively.

If I follow the procedure I used for numbers a) and b) I get 148 & 128 for 94 & 87.

Can someone explain to me how do I get to those results? Do I need to convert the byte to binary first and then proceed from there?

Thanks a lot in advance!

asked 36 secs ago

user3245995

69

Trouble understanding an exercise given the two's complement in hex format to convert into decimal format