Assembly Language - Converting Two's Complement Hexadecimal to its Base 10 Equivalent

Question

I am currently trying to figure out how to make the correct conversion from two's complement hexadecimal to its base 10 equivalent. We were given an assembly language program that adds and subtracts 32-bit integers and need to take the values of the registers and convert them.

Here is what I've attempted, however, my professor says I've made a critical error in my final calculation and I have no clue what.

Here is the register EAX value: FFFC0888

I then converted it from two's complement hexadecimal to hexadecimal: FFFC0888 -> 0003F778

And finally converted the hexadecimal to decimal. 0003F778 = (0 X 16^7) + (0 X 16^6) + (0 X 16^5) + (3 X 16^4) + (15 X 16^3) + (7 X 16^2) + (7 X 16^1) + (8 X 16^0) = 0+0+0+196608+61440+1792+112+8= 259960

My final answer of 259960 is not correct, can someone please help enlighten me as to what I'm doing wrong? Thank you!

is it because you failed to mention the answer as -259960 ?

chathux
– chathux

2015-05-07 03:11:24 +00:00
Commented May 7, 2015 at 3:11 — chathux
– chathux, Commented May 7, 2015 at 3:11

Ira Baxter · Accepted Answer · 2015-05-07 03:51:35Z

1

(@chathux hints at the answer).

You have a value X==FFFC0888_16. When you converted X to 0003F778 (by taking "twos complement"), you did so because you (should have) thought the X value was "negative" (the sign bit is set). The decimal value you produced 259960 is the correct magnitude, that is abs(X) == 259960_10. But the value of X is negative, so the correct answer is X == -259960_10.

If the value of X==7FFC0888_16, a positive value (sign bit is zero), you should not take the two's complement before you produce the decimal value.

answered May 7, 2015 at 3:51

Ira Baxter

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Comments

Machavity · Accepted Answer · 2017-05-15 18:31:29Z

1

Everything in your conversion was correct, except that:

0003F778 = -(0 + 0 + 0 + (3 X 16^4) + (15 X 16^3) + (7 X 16^2) + (7 X 16^1) + (8 X 16^0))

where 0003F778 is 2s complement integer data.

edited May 15, 2017 at 18:31

Machavity♦

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answered May 15, 2017 at 18:30

kjp

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