11
GENERAL OPERATION
The UT1750AR can operate in two modes. The first operating
mode is the Reduced Instruction Set Computer (RISC) mode;
the second is the MIL-STD-1750A Instruction Set Architecture
(ISA) emulation mode. The mode-select input pin (M1750)
determines the UT1750AR’s operating mode. M1750 must be
tied high to enable the MIL-STD-1750A ISAemulation mode
of operation; otherwise, an internal pull-down resistor pulls
M1750 low, selecting the RISC mode.
The UT1750AR has a Harvard architecture when it operates in
the RISC mode (M1750 = 0). A processor with a Harvard
architecture has two sets of address and data busses; one set
interfaces with instruction memory and the other set interfaces
with operand memory. This architecture allows the UT1750AR
to perform overlapping instruction fetch-and-execute bus cycles
that enhance processor throughput.
The UT1750AR’s reduced instruction set consists of 30 separate
instructions. The UT1750AR executes most of these
instructions in two clock cycles providing fast execution of
RISC-coded programs. All the UT1750AR’s processing
capabilities in the RISC mode are available to the system
programmer by using the companion RISC Assembler
(RASM)/Linker (RLNK), RISC Interactive Software Simulator
(IRSIM), and hardware development debug tools.
In the MIL-STD-1750A mode of operation (M1750 = 1), the
UT1750AR has a Von Neumann architecture. A processor with
a Von Neumann architecture has a common set of address and
data busses that make instructions and operand data available
to the processor.
The UT1750AR emulates the MIL-STD-1750A instruction set
when it has a specially programmed set of RISC PROMs. These
PROMs contain RISC-coded macros that correspond to each
MIL-STD-1750 instruction. When the UT1750AR fetches a
1750 instruction from memory, it decodes this instruction’s
opcode and generates an address for the RISC PROMs. This
address points to a RISC macro that, when executed, performs
the operation the 1750 instruction requires.
The high execution rate of the UT1750AR’s native RISC
language is also available when the UT1750AR is in the MIL-
STD-1750 mode of operation by using the MIL-STD-1750
Built-in-Function (BIF) opcode. The system designer can
develop a RISC macro for a specific function, such as power-
on self-test routines, built-in-test routines, signal-processing
routines, or any routine that requires real-time processing. The
UT1750AR executes this function when it encounters the BIF
in the MIL-STD-1750 program flow.
The RISC Mode of Operation
The configuration for the UT1750AR in the RISC mode of
operation is shown in figure 4. RISC is the default mode of
operation for the UT1750AR since the M1750 input is tied to
an internal pull-down resistor.
When the UT1750AR operates in the RISC mode, the system
designer stores the executable RISC program in RISC memory.
The UTMC RISC Assembler generates this executable RISC
program. All 20 of the RISC address lines can access a user-
defined program in RISC memory. This means the maximum
length of any RISC program is 1 mega- word.
Although the executable RISC program is all that is stored in
RISC memory, two RISC instructions allow the programmer to
manipulate the data in RISC memory. These instructions are the
Load Register from (RISC) Instruction Memory (LRI) and the
Store Register to (RISC) Instruction Memory (STRI).
When operating in the RISC mode, the UT1750AR first
generates an address on the RISC address bus for the instruction
it stores in the Primary Instruction Register (PIR). After the
UT1750AR stores the RISC instruction in the PIR, the
UT1750AR begins executing the instruction in the Instruction
Register (IR). If the present instruction in the IR requires only
internal processing, the UT1750AR does not exercise the
Operand Address and data busses. If, on the other hand, the
instruction in the IR requires some type of Operand Data, the
UT1750AR begins an Operand bus arbitration cycle midway
through the next processor clock cycle.
The Operand bus arbitration cycle begins with the UT1750AR
asserting the Bus Request (BRQ) signal. The UT1750AR
samples the Bus Grant (BGNT) and the Bus Busy ( BUSY)
signals on every falling edge of the processor clock. When the
UT1750AR detects that the previous bus controller has
relinquished control of the bus, the UT1750AR generates the
Bus Grant Acknowledge ( BGACK) signal signifying that it has
taken control of the bus.
After the UT1750AR has taken control of the bus, it generates
the Operand address and data. The Address Strobe (AS) and
Data Strobe (DS) signals indicate when the Operand address
and data are valid. If the UT1750AR is interfacing to slow
memory or other peripheral devices that require long memory-
access times, the Data Transfer Acknowledge (DTACK) signal
extends the memory cycle time. By holding off the assertion of
DTACK, the slow memory device lengthens the memory cycle
until it can provide data for the UT1750AR.