# Hexadecimal Multiplication using 2's complement

## Homework Statement

Perform the following operations using 2's complement method.
$FFFD_{16} * FFF1_{16}$

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## The Attempt at a Solution

$FFFD_{16}$ ----> (1's comp.) = $0002_{16}$ -----> (2's comp.) = $0003_{16}$
$FFF1_{16}$ ----> (1's comp.) = $000E_{16}$ -----> (2's comp.) = $000F_{16}$
$0003_{16} * 000F_{16} = D2_{16}$

But $FFFD_{16}$ ( negative ) * $FFF1_{16}$ ( negative ) , so the answer should be positive .
The answer is $= 0D2_{16}$ (positive ) .

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phinds
Gold Member
2019 Award
what is D2 base 16 in decimal? Does it sound like the right answer for 3x15?

what is D2 base 16 in decimal? Does it sound like the right answer for 3x15?
$2D_{16}$

phinds
Gold Member
2019 Award
$2D_{16}$
Did you not understand my question? What is that in DECIMAL base?

Did you not understand my question? What is that in DECIMAL base?
$D2_{16} = 13 * 16 + 2 * 16^0$
$= 210$
It's not the right answer for 3*15 .
I tried to solve the problem again and this is my work :
$3 * 15 = 45$ in decimal which is equal to $2D_{16}$
So, $FFFD_{16}$ ( negative ) * $FFF1$ (negative) = $02D_{16}$

phinds
Gold Member
2019 Award
$D2_{16} = 13 * 16 + 2 * 16^0$
$= 210$
It's not the right answer for 3*15 .
I tried to solve the problem again and this is my work :
$3 * 15 = 45$ in decimal which is equal to $2D_{16}$
So, $FFFD_{16}$ ( negative ) * $FFF1$ (negative) = $02D_{16}$
Yes, and can you do the actual arithmetic in base 16? (I wouldn't bother but I'm not sure if that's part of what you are supposed to do)

phinds
Gold Member
2019 Award
I solved it again and I got $2D_{16}$ not $D2_{16}$.
Yes, I deleted the post where I mistakenly said it 2D was wrong. I got the 2D confused with the D2

Yes, and can you do the actual arithmetic in base 16? (I wouldn't bother but I'm not sure if that's part of what you are supposed to do)
3 * F (15 in decimal) = 45 in decimal.
45 / 16 = 2.8125 ( 2 is the first digit , subtract 2 from 2.8125 and multiply the answer by 16 to get the second digit which is 13 in decimal ( D in base 16 ).

phinds
Gold Member
2019 Award
3 * F (15 in decimal) = 45 in decimal.
45 / 16 = 2.8125 ( 2 is the first digit , subtract 2 from 2.8125 and multiply the answer by 16 to get the second digit which is 13 in decimal ( D in base 16 ).
No, I mean can you do the original problem FFFD*FFF1 IN HEX not in decimal. I don't know that it's a requirement of the problem, I'm just asking if you CAN do it. Anyway, I think I've given you what help I can with this problem so I'll bow out now.

No, I mean can you do the original problem FFFD*FFF1 IN HEX
yes, but it's annoying and requires twice the register space and use of a rubber chicken to do it flat out instead of as complements.

NascentOxygen
Staff Emeritus
But $FFFD_{16}$ ( negative ) * $FFF1_{16}$ ( negative ) , so the answer should be positive .
The answer is $= 0D2_{16}$ (positive ) .