``The Interpreter'' is a micro-architecture that is intended
for a variety of uses including emulation of existing or hypothetical
machines and program profiling.
An emulator is written in microcode and instructions executed from the
microinstructions that are executed from the microstore give both
parallelism and fast execution.
More detailed review:
- A brief (1,000 word) history of microprogramming.
- (pg. 715) Suggested applications: emulation of existing or hypothetical
machines; direct execution of high-level languages; tuning
the instruction set to the application (by iterative profiling
and instruction-set change).
- (pg. 715) ``Emulation is defined in this paper as the ability
to execute machine language programs intended for one machine
(the mulated machine) on another machine (the host machine).
Within this broad definition, any machine with certain basic
cpabilities can emulate any other machine; however, a measure
of the efficiency of operation is generally implied when the
term emulation is used.
For example, ... a conventional computer [has poor] emulation
efficiency ... since for each machine language instruction of
the emulated machine there corresponds a sequence of machine
instructions to be executed on the host machine ... (called
simulation ... [Husson 70] and
turns out to be
significanly more efficent on micorprogrammable computers.
In a microprogrammed machine, the controls for performing the
fetching and execution fo the machine instructions of the
mulated machine consist of a sequence of microinstructions
which allows the increased efficiency.''
In short, as Deutsch and Schiffman point
out, you get hardware support for instruction fetch and
decode, which are typically multi-instruction operations in
- (pg. 717) Description of Interpreter features that help it
emulate a variety of machine architectures and instruction
- (pg. 719) ``The basic items necessary to define a machine and
hence emulate it are:
Note that you also need e.g. data formats, an exception model,
a device or other I/O model, ...
- Memory structure (registers, stacks, etc.),,
- Machine language format (0, 1, 2, 3 address)
including effective operand address calculation,
- Operation codes and their functional m eaning
(instructions for arithmetic, branching, etc.).''
- (pg. 719) ``The process of writing an emulation therefore,
involves the following analysis and the microprogramming of
these basic items:
- Mapping the registers (stacks, etc.) from the emulated
machine onto the host machine.
- Analysis of the machine language format and addressing
- Analysis of each operation code defined i the machine
- (pg. 719-720) ``All of the registers of the emulated machine
must be mapped onto the host machine; that is, each register
must have a corresponding register on the host machine. The
most frequently used registers are mapped onto the registers
within the Interpreter Logic Unit (e.g., registers A1, A2,
A3). The remaining registers are stored either in the main
system memory or in special high speed registers depending on
the desired emulatin speed [[Which, I assume, means ``do you
want R5 fast and R6 slow or R6 fast and R5 slow; it doesn't
make sense to me that they'd offer you slower emulation as a
feature --pardo]]. The machine language format may be 0, 1,
2, 3 address or variable length for increased code density,
and may involve indexing, indirection, relative addressing,
stacks and complex address conversions. Figure 14 shows the
general micrporogram requirements (MPM Map) and operating
procesures for the exmulation task.
You're probably already familiar with the concepts in this paper.
The paper describes the overall structure of a classic
this one happens to use microcode, but many of the same
features are the same when using normal machine code.
The opportunity with microcode (which tends to be poorly
stated in all of these papers) is that writable microcode
allows the use of a machine with a very fast and very flexible
but very space-consuming instruction set; microcode makes such
an instruction set useful by providing a fast
mechanism for mapping that instruction set to a denser
representation (the one stored in primary memory).
In the particular case of emulation, much of the interpreter
can be written directly in the low-density machine code and
can take advantage of that code's
flexibility and performance without being hurt by the low
- See also:
Deutsch's ST-80 VM,
written (largely) in Xerox Dorado microcode.
This review/summary by Pardo.
From instruction-set simulation and tracing