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Chapter 2 CPU Basics
Overview
Zero Flag (ZF)
Zero flag (ZF) is set to "1", when all bits are '0' in the operation result. Otherwise, zero flag is cleared to
"0".
Carry Flag (CF)
Carry flag (CF) is set to "1", when a carry from or a borrow to the MSB occurs. Carry flag is cleared to
"0", when no carry or borrow occurs.
Negative Flag (NF)
Negative flag (NF) is set to "1" when MSB is '1' and reset to "0" when MSB is '0'. Negative flag is used
to handle a signed value.
Overflow Flag (VF)
Overflow flag (VF) is set to "1", when the arithmetic operation results overflow as a signed value. Other-
wise, overflow flag is cleared to "0".
Overflow flag is used to handle a signed value.
Interrupt Mask Level (IM1 and IM0)
Interrupt mask level (IM1 and IM0) controls the maskable interrupt acceptance in accordance with the
interrupt factor interrupt priority for the interrupt control circuit in the CPU. The two-bit control flag
defines levels '0' to '3'. Level 0 is the highest mask level. The interrupt request will be accepted only
when the level set in the interrupt level flag (xxxLVn) of the interrupt control register (xxxICR) is higher
than the interrupt mask level. When the interrupt is accepted, the level is reset to IM1-IM0, and interrupts
whose mask levels are the same or lower are rejected during the accepted interrupt processing.
Maskable Interrupt Enable (MIE)
Maskable interrupt enable flag (MIE) enables/disables acceptance of maskable interrupts by the CPU's
internal interrupt acceptance circuit. A '1' enables maskable interrupts; a '0' disables all maskable inter-
rupts regardless of the interrupt mask level (IM1-IM0) setting in PSW.
This flag is not changed by interrupts.
Table 2-1-3 Interrupt Mask Level and Interrupt Acceptance
IM1 IM0
Mask level 0 0 0 High Non-maskable interrupt (NMI) only
Mask level 1 0 1 . NMI, Level 0
Mask level 2 1 0 . NMI, Level 0 to 1
Mask level 3 1 1 Low NMI, Level 0 to 2
Interrupt mask level
Priority Acceptable interrupt levels