Zilog Z08470 Manual de usuario Pagina 105

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UM008101-0601 Direct Memory Access
Reset and disable DMA interrupts
Enable DMA interrupts
Disable DMA interrupts
Load new starting addresses and block length
Continue prior address counting
Clear block length counter
Force the Ready condition
Read the status byte
Initiate a status-register read sequence
Clear status
Interrupt service routines on a Z80 CPU always end with a Return From
Interrupt (RETI or hex ED4D) instruction, which is now explained.
Figure 36. Interrupt On Ready (IOR) Latch
Return From Interrupt
At the end of an interrupt service routine, the Z80 CPU executes a return-
from-interrupt (RETI or hex ED4D) instruction. This returns the CPU from
the interrupt service routine.
The DMA also simultaneously decodes the RETI instruction, which it
recognizes on the data bus as an instruction (occurring when the DMAs
M1
input is Low). This causes at least one, and possibly two, events within
the DMA:
D
R
IOR
O
Disable DMA
CY
D-FLIP-FLOP
+5V
Interrupt Condition (Active RDY)
Enable after RETI
Reset and Disable Interrupts
*NOTE: This latch is only set when the Interrupt-On-Ready option is selected.
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