Find the value of the C flag after the execution of the foll

Find the value of the C flag after the execution of the following codes. LDI R20. $85 LD1 R21, $92 ADD R20, R21 LDI R16, $15 LDI R17, $72 ADD R16. R17 LDI R25, $F5 LDI R26, $52 ADD R25, R26 LDI R25. $FE INC R25 INC R25

Solution

Activity 2:

LD1 are simply the load instructions which will load the registers with values, so for example

LD1 R20, $85 means the value of register named R20 is 85 , less formally R20 =85

Here we are performing hexadecimal addition where the digits are from 0 to 15 and the digits from 10-15 are represented as A-F, the addition of hexadecimal can be understood with the following simple example:

   1 1

     A D D

+ D A D

______________

8 8 A

The first step is D + D which means 13 + 13 =26

to find the sum and carry we use 26 = 1 x 16 + 10(A)

so the answer to D+D = A with C=1

Here we get the final carry as C=1 as A+D+1 = 8 with carry =1

So the final carry is 1 in this case, similarly, we will get the following results:

a) 17 with C=1

b) 87 with C=0

c) 47 with C=1

d) INC means to add the value by 1, so we have to add 1 in R25 twice means FE+1 and again add 1 to it.

FE+1= FF with C=0

and FF +1 = 00 with C=1