| Floating Point Coprocessors Essay, Research Paper
Floating Point Coprocessors
The designer of any microprocessor would like to extend its instruction
set almost infinitely but is limited by the quantity of silicon available (not
to mention the problems of testability and complexity). Consequently, a real
microprocessor represents a compromise between what is desirable and what is
acceptable to the majority of the chip’s users. For example, the 68020
microprocessor is not optimized for calculations that require a large volume of
scientific (i.e. floating point) calculations. One method to significantly
enhance the performance of such a microprocessor is to add a coprocessor. To
increase the power of a microprocessor, it does not suffice to add a few more
instructions to the instruction set, but it involves adding an auxiliary
processor that works in parallel to the MPU (Micro Processing Unit). A system
involving concurrently operating processors can be very complex, since there
need to be dedicated communication paths between the processors, as well as
software to divide the tasks among them. A practical multiprocessing system
should be as simple as possible and require a minimum overhead in terms of both
hardware and software. There are various techniques of arranging a coprocessor
alongside a microprocessor. One technique is to provide the coprocessor with an
instruction interpreter and program counter. Each instruction fetched from
memory is examined by both the MPU and the coprocessor. If it is a MPU
instruction, the MPU executes it; otherwise the coprocessor executes it. It can
be seen that this solution is feasible, but by no means simple, as it would be
difficult to keep the MPU and coprocessor in step. Another technique is to equip
the microprocessor with a special bus to communicate with the external
coprocessor. Whenever the microprocessor encounters an operation that requires
the intervention of the coprocessor, the special bus provides a dedicated high-
speed communication between the MPU and the coprocessor. Once again, this
solution is not simple. There are more methods of connecting two (or more)
concurrently operating processors, which will be covered in more detail during
the specific discussions of the Intel and Motorola floating point coprocessors.
Motorola Floating Point Coprocessor (FPC) 68882
The designers of the 68000-family coprocessors decided to implement
coprocessors that could work with existing and future generations of
microprocessors with minimal hardware and software overhead. The actual approach
taken by the Motorola engineers was to tightly couple the coprocessor to the
host microprocessor and to treat the coprocessor as a memory-mapped peripheral
lying inside the CPU address space. In effect, the MPU fetches instructions from
memory, and, if an instruction is a coprocessor instruction, the MPU passes it
to the coprocessor by means of the MPU’s asynchronous data transfer bus. By
adopting this approach, the coprocessor does not have to fetch or interpret
instructions itself. Thus if the coprocessor requires data from memory, the MPU
must fetch it. There are advantages and disadvantages to this design. Most
notably, the coprocessor does not have to deal with, for example, bus errors, as
all fetching is performed by the host MPU. On the other hand, the FPC can not
act as a bus master (making it a non-DMA device), making memory accesses by the
FPC slower than if it were directly connected to the address and data bus.
In order for the coprocessor to work as a memory mapped device, the
designers of the 68000 series of MPU’s had to set aside certain bit patterns to
represent opcodes for the FPC. In the case of the 68000’s, the FPC is accessed
through the opcode 1111(2). This number is the same as ?F’ in hexadecimal
notation, so this bit pattern is often referred to as the F-line.
Interface
The 68882 FPC employs an entirely conventional asynchronous bus
interface like all 68000 class devices, and absolutely no new signals whatsoever
are required to connect the unit to an MC 68020 MPU. The 68882 can be configured
to run under a variety of different circumstances, including various sized data
buses and clock speeds. What follows is a diagram of connections necessary to
connect the 68882 to a 68020 or 68030 MPU using a 32-bit data path.
As mentioned previously, all instructions for the FPC are of the F-line
format, that is, they begin with the bit pattern 1111(2). A generic coprocessor
instruction has the following format: the first four bits must be 1111. This
identifies the instruction as being for the coprocessor. The next three bits
identify the coprocessor type, followed by three bits representing the
instruction type. The meaning of the remaining bits varies depending on the
specific instruction.
Coprocessor Operation
When the MPU detects an F-line instruction, it writes the instruction
into the coprocessors memory mapped command register in CPU space. Having sent a
command to the coprocessor, the host processor reads the reply from the
coprocessor’s response register. The response could, for example, instruct the
processor to fetch data from memory. Once the host processor has complied with
the demands from the coprocessor, it is free to continue with instruction
processing, that is, both the processor and coprocessor act concurrently. This
is why system speed can be dramatically improved upon installation of a
coprocessor.
MC 68882 Specifics
The MC 68882 floating point coprocessor is basically a very simple
device, though it’s data manual is nearly as thick as that of the MC 68000. This
complexity is due to the IEEE floating point arithmetic standards rather than
the nature of the FPC. The 68882 contains eight 80-bit floating point data
registers, FP0 to FP7, one 32-bit control register, FPCR, and one 32-bit status
register, FPSR. Because the FPC is memory mapped in CPU space, these registers
are directly accessible to the programmer within the register space of the host
MPU. In addition to the standard byte, word and longword operations, the FPC
supports four new operand sizes: single precision real (.S), double precision
real (.D), extended precision real (.X) and packed decimal string (.P). All on-
chip calculations take place in extended precision format and all floating point
registers hold extended precision values. The single real and double real
formats are used to input and output operands. All three real floating point
formats comply with the corresponding IEEE floating point number standards. The
FPC has built in functions to convert between the various data formats added by
the unit, for example a register move with specified operand type (.P, .B, etc).
The 68882 FPC has a significant instruction set designed to satisfy many
number-crunching situations. All instructions native to the FPC start with the
bit pattern 1111(2) to show that the instruction deals with floating point
numbers. Some instructions supported by the FPC include FCOSH, FETOX, FLOG2,
FTENTOX, FADD, FMUL and FSQRT. There are many more instructions available, but
this excerpt demonstrates the versatility of the 68882 unit.
One of the registers within the FPC is the status register. It is very
similar in function to the status register in a CPU; it is updated to show the
outcome of the most recently executed instruction. Flags within the status
register of the FPC include divide by zero, infinity, zero, overflow, underflow
and not a number. Some of the conditions signaled by the status register of the
FPC (for example divide by zero) require an exception routine to be executed, so
that the user is informed of the situation. These exceptions are stored and
executed within the host MPU, which means that the FPC can be used to control
loops and tests within user programs ? further extending the functionality of
the coprocessor.
Intel Math Coprocessor 80387 DX
In many respects, the Intel 80387 math coprocessor (MCP) is very similar
to the MC 68882. Both designs were influenced by such factors as cost, usability
and performance. There are, however, subtle differences in the designs of the
two units.
Firstly, I shall discuss the similarities between the designs followed
by differences. Like the 68882, the 80387 requires no additional hardware to be
connected to a 80386. It is a non-DMA device, having no direct access to the
address bus of the motherboard. All memory and I/O is handled by the CPU, which
upon detection of a MCP instruction passes it along to the MCP. If additional
memory reads are necessary to load operands or data, the MCP instructs the CPU
to perform these actions. This design, although reducing MCP performance when
compared to a direct connection to the address bus, significantly decreases
complexity of the MCP as no separate address decoding or error handling logic is
necessary. The connection between the CPU and the MCP instruction is via a
synchronous bus, while internal operation of the MCP can run asynchronously
(higher clockspeed). Moreover, the three functional units of the MCP can work in
parallel to increase system performance. The CPU can be transferring commands
and data to the MCP bus control logic while the MCP floating unit is executing
the current instruction. Similar to the 68882, the 80387 has a bit pattern
(11011(2)) reserved to identify instructions intended for it. Also, the
registers of the MCP are memory mapped into CPU address space, making the
internal registers of the MCP available to programmers.
Internally, the 80387 contains three distinct units: the bus control
logic (BCL), the data interface and control unit and the actual floating point
unit. The data interface and control unit directs the data to the instruction
decoder. The instruction decoder decodes the ESC instructions sent to it by the
CPU and generates controls that direct the data flow in the instruction buffer.
It also triggers the microinstruction sequencer that controls execution of each
instruction. If the ESC instruction is FINIT, FCLEX, FSTSW, FSTSW AX, or FSTCW,
the control unit executes it independently of the FPU and the sequencer. The
data interface and control unit is the unit that generates the BUSY?, PEREQ and
ERROR? signals that synchronize Intel 387 DX MCP activities with the Intel 80386
DX CPU. It also supports the FPU in all operations that it cannot perform alone
(e.g. exceptions handling, transcendental operations, etc.).
The FPU executes all instructions that involve the register stack,
including arithmetic, logical, transcendental, constant, and data transfer
instructions. The data path in the FPU is 84 bits wide (68 significant bits, 15
exponent bits, and a sign bit) which allows internal operand transfers to be
performed at very high speeds.
Interface
The MCP is connected to the MPU via a synchronous connection, while the
numeric core can operate at a different clock speed, making it asynchronous. The
following diagram will clarify this.
The following diagram shows the specific connections necessary between
the 80386 MPU and the 80387 MCP.
A typical coprocessor instruction must begin with the bit pattern
11011(2) to identify the instruction for the coprocessor. The bus control logic
of the MCP (BCL) communicates solely with the CPU using I/O bus cycles. The BCL
appears to the CPU as a special peripheral device. It is special in one
important respect: the CPU uses reserved I/O addresses to communicate with the
BCL. The BCL does not communicate directly with memory. The CPU performs all
memory access, transferring input operands from memory to the MCP and
transferring outputs from the MCP to memory.
Coprocessor Operation
When the CPU detects the arrival of a coprocessor instruction, it writes the
instruction into the coprocessors memory mapped command register in CPU space.
Having sent a command to the coprocessor, the host processor reads the reply
from the coprocessor’s signals. The response could, for example, instruct the
processor to fetch data from memory. Once the host processor has complied with
the demands from the coprocessor, it is free to continue with instruction
processing, that is, both the processor and coprocessor act concurrently. This
is why system speed can be dramatically improved upon installation of a
coprocessor.
80387 Specifics
Just like the MC 68882 floating point coprocessor, the Intel 80387 is basically
a very simple device. Like any reasonable math coprocessor, it conforms to the
IEEE standards of floating point number representations. The 80387 contains
eight 82-bit floating point data registers (including a 2-bit tag field), R0 to
R7, one 16-bit control register, one 16-bit status register and a tag word (that
contains the tag fields for the eight data registers). The MCP also indirectly
uses the 48-bit instruction and data pointer registers of the 80386 host
processor, even though these are external to the unit. Because the FPC is memory
mapped in CPU space, these registers are directly accessible to the programmer
within the register space of the host MPU. In addition to the standard word,
short and long (16, 32 and 64-bit) integer operations, the MCP supports four new
operand sizes: single precision real, double precision real, extended precision
real and packed binary coded decimal strings. All on-chip calculations take
place in extended precision format and all floating point registers hold
extended precision values. The single real and double real formats are used to
input and output operands. All three real floating point formats comply with the
corresponding IEEE floating point number standards. The MCP has built in
functions to convert between the various data formats added by the unit.
The 80387 has a significant instruction set designed to satisfy many
number-crunching situations. All instructions native to the MCP start with the
bit pattern 11011(2) to show that the instruction should be directed to the
coprocessor. Some (of the over 70) instructions supported by the MCP are FCOMP,
FDIV, FSQRT, FSINCOS, FINIT. There are many more instructions available, but
this excerpt demonstrates the versatility of the 80387 unit, which is very
similar to that of the 68882 unit.
One of the registers within the MCP is the status register. Just like
for the 68882, the status register shows the outcome of the most recently
executed instruction. Flags within the status register of the FPC include divide
by zero, infinity, zero, overflow, underflow and invalid operation. Some of the
conditions signaled by the status register of the FPC (for example divide by
zero) require an exception routine to be executed by the host MPU, so that the
user is informed of the situation. These exceptions are stored and executed
within the host MPU, which means that the MCP can again be used to control loops
and tests within user programs ? further extending the functionality of the
coprocessor. The Intel 80387 DX MCP register set can be accessed either as a
stack, with instructions operating on the top one or two stack elements, or as a
fixed register set, with instructions operating on explicitly designated
registers. The TOP field in the status word identifies the current top-of-stack
register. A “push” operation decrements TOP by one and loads a value into the
new TOP register. A “pop” operation stores the value from the current top
register and then increments TOP by one. Like the 80386 DX microprocessor stacks
in memory, the MCP register stack grows “down” toward lower-addressed
registers. Instructions may address the data registers either implicitly or
explicitly. The explicit register addressing is also relative to TOP. A notable
feature of the 80387 is the addition of a tag field of 2 bits to each of the
eight floating point registers. The tag word marks the content of each numeric
data register, as Figure 2.1 shows. Each two-bit tag represents one of the eight
numeric registers. The principal function of the tag word is to optimize the
MCP’s performance and stack handling by making it possible to distinguish
between empty and nonempty register locations. It also enables exception
handlers to check the catchword of a stack location without the need to perform
complex decoding of the actual data.
Evaluation of the two Coprocessor
I started this paper thinking that the Motorola math coprocessor had to
be better in design, implementation and features than its Intel counterpart.
Throughout my research I came to realize that my opinions were based on nothing
but myths. In many respects the two coprocessors are very similar to each other,
while in other respects the coprocessors differ radically in design and
implementation. I will sum up the points I consider most important.
1. Intel uses a synchronous bus between the CPU and the MCP, while the actual
internal floating unit can run asynchronously to this. This increases complexity
of the design as synchronization logic must exist between the two processors,
but like this the floating point unit can run at a higher clock speed than the
CPU upon installation of a dedicated clock generator. 2. The (logical, not
physical) addition of tag fields to the data registers in the 80387 to signal
certain conditions of the data registers makes certain operations that support
tags much faster, as certain information does not need to be decoded as it is ?
cached? in the tag fields. 3. The 80387 can use its registers either in stack
mode or absolute addressing mode. Though some operations require stack
addressing, this feature adds a little more flexibility to the MCP (even though
the stack operations might be a legacy from the 8087 or 80287).
In most other fields, the coprocessors are equals. They have the same number of
data registers, both add their own instruction set and registers to programmers
in a transparent fashion and both support the same IEEE numeric representation
standards. Probably both coprocessors have similar processing power at equal
clockspeed as well. Even though the Motorola coprocessor seems to be superior by
name, I have to admit that the 80387 gets my vote for more flexibility and
thoughtful optimizations (tags).
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