UPD75518 NEC, UPD75518 Datasheet - Page 36

• Manufacturer Part Number: UPD75518
• Description: 4 BIT SINGLE-CHIP MICROCOMPUTER
• Manufacturer: NEC

36
(1) Carry flag (CY)
(2) Skip flags (SK2, SK1, SK0)
(3) Interrupt status flag (IST1, IST0)
Instruction dedicated to
carry flag manipulation
Bit transfer instruction
Bit Boolean instruction
Interrupt handling
The carry flag is a 1-bit flag used to store overflow or underflow occurrence information when an arithmetic
operation with a carry (ADDC, SUBC) is executed.
The carry flag also has the function of a bit accumulator, and therefore can be used to store the result of
a Boolean operation performed on the CY and bit at a specified data memory bit address.
The carry flag is manipulated using special instructions, independently of the other PSW bits.
A RESET signal occurrence causes the carry flag to be undefined.
Remark
Example
The skip flags are used to store skip status, and are automatically set or reset when the CPU executes an
instruction.
The user cannot directly manipulate these flags as operands.
The interrupt status flag is a 2-bit flag used to store the status of processing being performed. (For detailed
information, see Table 5-3.)
mem*.bit represents the following three addressing modes:
• fmem.bit
• pmem.@L
• @H+mem.bit
Bit 3 at address 3FH is ANDed with P33, then the result is output to P50.
MOV
MOV1
AND1
MOV1
CY, @H+0FH.3 ; CY
PORT5.0, CY
H, #3H
CY, PORT3.3
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Instruction (mnemonic)
SET1 CY
CLR1 CY
NOT1 CY
SKT CY
MOV1 mem*.bit,CY
MOV1 CY,mem*.bit
AND1 CY,mem*.bit
OR1 CY,mem*.bit
XOR1 CY,mem*.bit
Interrupt execution
RETI
Table 3-3 Carry Flag Manipulation Instructions
; Set high-order 4 bits in register H
; CY
; P50
Sets CY to 1.
Clears CY to 0.
Inverts the contents of CY.
Skips if CY is set to 1.
Transfers the contents of CY to a specified bit.
Transfers the contents of a specified bit to CY.
ANDs, ORs, or XORs CY with the contents of a specified bit, then
sets the result in CY.
Saves CY and all other PSW bits to stack memory in parallel.
Restores CY together with the other PSW bits from stack memory.
Bit 3 at 3FH
CY
CY
P33
Carry flag operation/processing
PD75518(A)