Zilog Z80380 Manual do Utilizador descarregar pdf (Página 147)

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Z380

™

USER'S MANUAL

ZILOG

DC-8297-03

OTDM

OUTPUT DECREMENT MEMORY

OTDM

Operation: (C) ← (HL)

C ← C – 1

B ← B – 1

HL ← HL – 1

This instruction is used for block output of strings of data to on-chip peripherals. No external

I/O transaction will be generated as a result of this instruction, although the I/O address will

appear on the address bus and the write data will appear on the data bus while this internal

write is occurring. The peripheral address is placed on the low byte of the address bus and

zeros are placed on all other address lines. The byte of data from the memory location

addressed by the HL register is loaded to the on-chip I/O port addressed by the C register.

The C register, holding the port address, is decremented by one to select the next output

port. The B register, used as a counter, is then decremented by one. The HL register is then

decremented by one, thus moving the pointer to the next source for the output.

Flags: S: Set if the result of decrementing B is negative; cleared otherwise

Z: Set if the result of decrementing B is zero; cleared otherwise

H: Set if there is a borrow from bit 4 during the decrement of the B register; cleared

otherwise

P: Set if the result of the decrement of the B register is even; cleared otherwise

N: Set if the most significant bit of the byte transferred was a 1; cleared otherwsie

C: Set if there is a borrow from the most significant bit during the decrement of the B

Addressing Execute

Mode Syntax Instruction Format Time Note

OTDM 11101101 10001011 2+r+o

1 2 ... 142 143 144 145 146 147 148 149 150 151 152 ... 267 268

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Zilog Z80380 Manual do Utilizador Página 147