Question

The following 3 instances of code was provided by my embedded systems course. Can you explain what is happening in each code segment?

=== code 1 ===

void foo() {
uint8_t a=2;
uint8_t b[]={b0, b1, b2}; // They are the last three digits of your A#
uint8_t* c=b;
uint8_t* d=&a;
}


=== code 2 ===

__extension__ typedef struct tagT1CONBITS {
union {
struct {
unsigned :1;
unsigned TCS:1;
unsigned TSYNC:1;
unsigned :1;
unsigned TCKPS:2;
unsigned TGATE:1;
unsigned :6;
unsigned TSIDL:1;
unsigned :1;
unsigned TON:1;
};
struct {
unsigned :4;
unsigned TCKPS0:1;
unsigned TCKPS1:1;
};
uint8_t T1CON[2];
};
} T1CONBITS;
volatile T1CONBITS foo;


=== code 3 ===

b6: 00 e0 ldi r16, 0x00
b8: 10 e0 ldi r17, 0x00
ba: 0c c0 rjmp .+24
bc: 81 2f mov r24, r17
be: 90 e0 ldi r25, 0x00
c0: 29 81 ldd r18, Y+1
c2: 3a 81 ldd r19, Y+2
c4: 82 0f add r24, r18
c6: 93 1f adc r25, r19
c8: fc 01 movw r30, r24
ca: 80 81 ld r24, Z
cc: 08 0f add r16, r24
ce: 81 2f mov r24, r17
d0: 11 e0 ldi r17, 0x01
d2: 18 0f add r17, r24
d4: 13 30 cpi r17, 0x03
d6: 90 f3 brcs .-28

Answer 1

=== code 1 ===

void foo() {
uint8_t a=2;
uint8_t b[]={b0, b1, b2}; // They are the last three digits of your A#
uint8_t* c=b;
uint8_t* d=&a;
}

• in the above code 1 section, unit_8_t is integer of 1 byte. note: when just int is declared it generally occupies 4 bytes of memory across all platforms.
• The function foo() is defined , line number 1 tells, variable a of type int(1 byte) is used and value 2 has been assigned to variable a
• line number 2 tells, integer(1 byte) array b is defined and the 3 integers b0,b1 and b2 are stored in array b.
• line number 3 tells, the pointer c of type integer(1 byte) is declared and integer array b is assigned to pointer c. this suggests the pointer c is storing the address of first element of array that is b0.
• line number 4 tells, the pointer d of type integer(1 byte) is declared and storing the address of variable a. here &a suggests the address of variable a.

=== code 2 ===

__extension__ typedef struct tagT1CONBITS {
union {
struct {
unsigned :1;
unsigned TCS:1;
unsigned TSYNC:1;
unsigned :1;
unsigned TCKPS:2;
unsigned TGATE:1;
unsigned :6;
unsigned TSIDL:1;
unsigned :1;
unsigned TON:1;
};
struct {
unsigned :4;
unsigned TCKPS0:1;
unsigned TCKPS1:1;
};
uint8_t T1CON[2];
};
} T1CONBITS;
volatile T1CONBITS foo;

• in this code snippet, typedef is a reserved keyword in the programming languages C and C++. It is used to create an additional name for another data type, but does not create a new type i.e new name to exisiting data type.for example, struct tagT1CONBITS is renamed to T1CONBITS with the help of typedef.
• struct, union are are derived data types these contain int, float, double,char type varibles etc.
• unsigned TCS:1 here ":value" represents bitfield and it is used to manage memory effciently. bitfiled is maximum number of bits that varible can use for getting value. for example, unsigned TCS:1 here the variable can take value 0 or 1 .it can't take more than value 1 becuase maximum number of bits allowed is 1.
• volatile keyword tells value of variable can be changed at any time.