Intel 80C196NU, 8XC196NP, Microcontroller manual Psw, Bit Function Number Mnemonic

REGISTERS

PSW

PSW (Continued) no direct access

The processor status word (PSW) actually consists of two bytes. The high byte is the status word, which is described here; the low byte is the INT_MASK register. The status word contains one bit (PSW.1) that globally enables or disables servicing of all maskable interrupts, one bit (PSW.2) that enables or disables the peripheral transaction server (PTS), and six Boolean flags that reflect the state of a user’s program.

The status word portion of the PSW cannot be accessed directly. To access the status word, push the value onto the stack (PUSHF), then pop the value to a register (POP test_reg). The PUSHF and PUSHA instructions save the PSW in the system stack and then clear it; POPF and POPA restore it.

15

Z N V VT

7

8

C PSE I ST

0

		See INT_MASK on page C-25
Bit	Bit	Function
Number	Mnemonic
4	VT	Overflow-trap Flag
		This flag is set when the overflow flag is set, but it is cleared only by the
		CLRVT, JVT, and JNVT instructions. This allows testing for a possible
		overflow at the end of a sequence of related arithmetic operations, which
		is generally more efficient than testing the overflow flag after each
		operation.
3	C	Carry Flag
		This flag is set to indicate an arithmetic carry or the last bit shifted out of
		an operand. It is cleared if a subtraction operation generates a borrow.
		Normally, the result is rounded up if the carry flag is set. The sticky bit
		flag allows a finer resolution in the rounding decision. (See the PSW flag
		descriptions in Appendix A for details.)
2	PSE	PTS Enable
		This bit globally enables or disables the peripheral transaction server
		(PTS). The EPTS instruction sets this bit; DPTS clears it.
		1 = enable PTS
		0 = disable PTS
1	I	Interrupt Disable (Global)
		This bit globally enables or disables the servicing of all maskable
		interrupts. The bits in INT_MASK and INT_MASK1 individually enable or
		disable the interrupts. The EI instruction sets this bit; DI clears it.
		1 = enable interrupt servicing
		0 = disable interrupt servicing
0	ST	Sticky Bit Flag
		This flag is set to indicate that, during a right shift, a “1” was sh ifted into
		the carry flag and then shifted out. It can be used with the carry flag to
		allow finer resolution in rounding decisions.

C-35