MAS281 Dynex, MAS281 Datasheet - Page 4

• Manufacturer Part Number: MAS281
• Description: MIL-STD-1750A Microprocessor
• Manufacturer: Dynex

MAS281
with the exact number determined by the type of operation
being performed. Internal processor operations, excluding
internally decoded XIO commands, require either five or six
oscillator cycles, the former associated with sequential
microcode execution and the latter with microcode branches.
Internally decoded XIO commands require a minimum of six
oscillator cycles to complete. External processor operations
require a minimum of five oscillator cycles to complete.
an internally decoded XIO command is detected An external
ready interface is provided which allows external machine
cycles to be extended when interfacing with slow devices. The
external ready signal is provided by external logic and must be
asserted in order to conclude the machine cycle.
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The internal ready signal is generated by the IU whenever
A machine cycle requires five or more oscillator cycles
Field
CS
R
PS
AS
0
CS
Figure 3: Status Word Format
Bits
0
1
2
3
4 - 7
8 - 11
12 - 15
3 4
R
7 8
CONDITION STATUS:
C-
addition or no borrow from
a subtraction
P-
Z-
N-
RESERVED
PROCESSOR STATE:
(a)- Memory access key
code
(b)- Privileged instruction
enable
ADDRESS STATE:
Page register sets for
expanded memory
addressing
PS
11 12
Description
Carry from an
Result >0
Result = 0
Result<0
AS
15
1.1.7 OPERAND TRANSFER REGISTERS
serve to buffer transfers between the data path and the
Address/Data (AD) bus. These registers are used under
microcode control and are not directly accessible by software.
A description of the use of these registers during memory and
l/O operations is provided in section 3.0.
1.1.8 INSTRUCTION FETCH REGISTERS
Instruction B (IB) registers allow sequential instruction fetches
to be performed without the assistance of the ALU. The IC
register, which holds a 16-bit address and points to the next
instruction to be fetched, is loaded indirectly via reset, jump, or
branch operations. Once loaded, it uses a dedicated counter to
sequence from one instruction to the next. IA and IB serve as
an instruction pipeline with IA storing the next instruction to be
executed. Dl also plays a role by storing any immediate
operands. Use of these registers during instruction fetches is
described in section 3.0.
1.1.9 MICROCODE CONTROL LOGIC
As depicted in Figure 2, microinstructions are provided by the
CU over the M bus, buffered by the Execution (E) register, and
decoded to generate various control signals.
1.2 CONTROL UNIT (CU)
operations. It features a microsequencer, a microcode storage
ROM, and an instruction mapping ROM. A brief description of
these features follows:
1.2.1 MICROSEQUENCER
microcode ROM accesses. The microsequencer features a
program counter (PC) which points to the next sequential
microinstruction, a program counter save register (SV) to save
return addresses for microsubroutines, address increment
logic (INCR), instruction pipeline registers (IA and IB), a next
address multiplexer, a loop counter (C), and various
miscellaneous systems.
MILSTD-1750A, or macro, instruction by stepping through its
corresponding microcode sequence. If the macroinstruction is
a conditional, the CS bits of the status word will be interrogated
to determine the necessary course of action. At the completion
of each macroinstruction, the microsequencer checks to see if
a Hold request or an interrupt is pending. If so, the
microsequencer will branch to the appropriate microinstruction
sequence. If not, the microsequencer begins sequencing the
next macroinstruction.
the EU state sequencer. Each processor machine cycle
corresponds to the execution of a single microinstruction.
The Address (A), Data In (Dl), and Data Out (DO) registers
The Instruction Counter (IC), Instruction A (IA), and
All EU operations are performed under microcode control.
The CU provides microprogrammed control of all MAS281
This 12-bit wide microcode address generator controls all
The microsequencer controls the execution of each
Note that the microsequencer is itself under the control of