Question

Four 8-bit 2's complement (signed) integers A, B, C, and D are packed into the 32-bit word stored in the memory of an ARM processor at address 0x36F0:

Extract the 8-bit integer A into the 32-bit register Ra, the 8-bit register B into the 32-bit register Rb,the 8-bit register C into the 32-bit register Rc, and the 8-bit register D into the 32-bit register Rd.

Hint: in ARM an address refers to a byte, not a word

Answer 1

unsigned long ptr=0x36F0;
long TwosCompVal=0,temp;
char TempA,TempB,TempC,TempD;
/*
Pack the Two's complement
*/

temp = A; TwoCompVal = temp << 24;
temp = B; TwoCompVal = TwoCompVal + (temp << 16);
temp = C; TwoCompVal = TwoCompVal + (temp << 8);
temp = D; TwoCompVal = TwoCompVal + temp;

//Now TwoCompVal = ABCD

/*
Unpack the TwoCompVal
*/

TwoCompVal = *ptr; //Retrieve Data from memory location 0x36F0
Temp = (TwoCompVal && 0x000000FF); TempD = temp;
Temp = (TwoCompVal && 0x0000FF00)>>8; TempC = temp;
Temp = (TwoCompVal && 0x00FF0000)>>16; TempB = temp;
Temp = (TwoCompVal && 0xFF000000)>>23; TempA = temp;