Describe flags

There are 4 types of flag, sign S, zero Z, overflow V and carry C.
These flags are corded as 4-bit bus F.
The bit order of F is S, Z, V, C from most significant bit.
The changes of flags are individually differ in each operation.

Arithmetic operations

Operation	S	Z	V	C
Addition (ADD)	'1' when result is minus.	'1' when result is zero.	'1' when result is incorrect in signed addition.	'1' when carry occurs in unsigned addition.
Subtract (SUB)	'1' when result is minus.	'1' when result is zero.	'1' when result is incorrect in signed subtraction.	'1' when borrow occurs in unsigned subtraction.

Logical operations

Operations	S	Z	V	C
Logical OR (OR)	same as most significant bit	'1' when result is zero	always '0'	always '0'
Logical NAD (AND)	same as most significant bit	'1' when result is zero	always '0'	always '0'
Logical NOT (NOT)	same as most significant bit	'1' when result is zero	always '0'	always '0'

Shift and exchange operations

Operation	S	Z	V	C
Arithmetic shift right (ASR)	'1' when result is minus.	'1' when result is zero	always '0'	same as least significant bit
Arithmetic shift left (ASL)	'1' when result is minus.	'1' when result is zero	'1' when bit 7 and bit 6 of operand are different value	same as bit 6 od operand
Nibble swap (SWP)	same as most significant bit	'1' when result is zero	always '0'	always '0'

Each flag is generated as follows;

• Sign flag; same as most significant bit
• Zero flag: '1' when result is zero
• Carry flag: It represent the carry in unsigned addition, the borrow in unsigned subtraction, flowed bit in shift operations.
• Overflow flag: '1' when the result is incorrect in signed operations;
  1. In ADD operation, set '1' when sign of augend and sign of addend are same value, and sign of augend and sign of addend are different value.
  2. In SUB operation, set '1' when sign of minuend and sign of subtrahend are different value, and sign of minuend and sign of result are different value .
  3. In ASL operation, set '1' when bit 7 and bit 6 of operand are different value.

  reg V;				// for overflow

  always @( ... or ... or ... or ... )	// overflow calculation
    case ( CB )
    `IADD   : V <= ( ... ) ? 1 : 0;
    `ISUB   : V <= ( ... ) ? 1 : 0;
    `ASL    : V <= ( ... ) ? 1 : 0;
    default : V <= 0;
    endcase

  assign F[3] = ...... ;	// Sign flag
  assign F[2] = ...... ;	// Zero flag
  assign F[1] = V ;		// Overflow flag
  assign F[0] = ...... ;	// Carry flag

Next, Functional simulation for verification.