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PACE1750A

Document # MICRO-3 Rev. C

SIGNAL DESCRIPTIONS (Continued)

BUS CONTROL

Mnemonic

Name

Description

D/

I

Data or instruction

An output signal that indicates whether the current bus cycle access is for

Data (HIGH) or Instruction (LOW). It is three-state during bus cycles not

assigned to the CPU. This line can be used as an additional memory

address bit for systems that require separate data and program memory.

R/

W

Read or write

An output signal that indicates direction of data flow with respect to the

current bus master. A HIGH indicates a read or input operation and a

LOW indicates a write or output operation. The signal is three-state during

bus cycles not assigned to the CPU.

M/

IO

Memory or I/O

An output signal that indicates whether the current bus cycle is memory

(HIGH) or I/O (LOW). This signal is three-state during bus cycles not

assigned to the CPU.

STRBA

Address strobe

An active HIGH output that can be used to externally latch the memory or

I/O address at the HIGH-to-LOW transition of the strobe. The signal is

three-state during bus cycles not assigned to the CPU.

RDYA

Address ready

An active HIGH input that can be used to extend the address phase of a

bus cycle. When RDYA is not active, wait states are inserted by the device

to accommodate slower memory or I/O devices.

STRBD

Data strobe

An active LOW output that can be used to strobe data in memory and XIO

cycles. This signal is three-state during bus cycles not assigned to the

CPU.

RDYD

Data ready

An active HIGH input that extends the data phase of a bus cycle. When

RDYD is not active, wait states are inserted by the device to accommodate

slower memory or I/O devlces.

INFORMATION BUS

Mnemonic

Name

Description

Information bus

A bidirectional time-multiplexed address/data bus that is three-state

STATUS BUS

Mnemonic

Name

Description

Access key

Outputs used to match the access lock in the MMU for memory accesses

(a mismatch will cause the MMU to pull the

MEM PRT ER signal LOW),

and also indicates processor state (PS). Privileged instructions can be

executed with PS = 0 only. These signals are three-state during bus

cycles not assigned to the CPU.

Address state

Outputs that select the page register group in the MMU. It is three-state

during bus cycles not assigned to the CPU. [These outputs together with

D/

I can be used to expand the device direct addressing space to 4

MBytes, in a nonprotected mode (no MMU)].

However, using this

addressing mode may produce situations not specified in MIL-STD-1750.