Intel iAPX 432

Intel's 432 project started in 1975, a year after the 8-bit Intel 8080 was completed and a year before their 16-bit 8086 project began. The 432 project was initially named the 8800,[5] as their next step beyond the existing Intel 8008 and 8080 microprocessors. This became a very big step. The instruction sets of these 8-bit processors were not very well fitted for typical Algol-like compiled languages. However, the major problem was their small native addressing ranges, just 16K for 8008 and 64K for 8080, far too small for many complex software systems without using some kind of bank switching, memory segmentation, or similar mechanism (which was built into the 8086, a few years later on). Intel now aimed to build a sophisticated complete system in a few LSI chips, that was functionally equal to or better than the best 32-bit minicomputers and mainframes requiring entire cabinets of older chips. This system would support multiprocessors, modular expansion, fault tolerance, advanced operating systems, advanced programming languages, very large applications, ultra reliability, and ultra security. Its architecture would address the needs of Intel's customers for a decade.[10]

The iAPX 432 development team was managed by Bill Lattin, with Justin Rattner as the lead engineer[11][12][13] (although one source[1] states that Fred Pollack was the lead engineer). (Rattner would later become CTO of Intel.) Initially the team worked from Santa Clara, but in March 1977 Lattin and his team of 17 engineers moved to Intel's new site in Portland.[12] Pollack later specialized in superscalarity and became the lead architect of the i686 chip Intel Pentium Pro.[1]

It soon became clear that it would take several years and many engineers to design all this. And it would similarly take several years of further progress in Moore's Law, before improved chip manufacturing could fit all this into a few dense chips. Meanwhile, Intel urgently needed a simpler interim product to meet the immediate competition from Motorola, Zilog, and National Semiconductor. So Intel began a rushed project to design the 8086 as a low-risk incremental evolution from the 8080, using a separate design team. The mass-market 8086 shipped in 1978.

The 8086 was designed to be backward-compatible with the 8080 in the sense that 8080 assembly language could be mapped on to the 8086 architecture using a special assembler. Existing 8080 assembly source code (albeit no executable code) was thereby made upward compatible with the new 8086 to a degree. In contrast, the 432 had no software compatibility or migration requirements. The architects had total freedom to do a novel design from scratch, using whatever techniques they guessed would be best for large-scale systems and software. They applied fashionable computer science concepts from universities, particularly capability machines, object-oriented programming, high-level CISC machines, Ada, and densely encoded instructions. This ambitious mix of novel features made the chip larger and more complex. The chip's complexity limited the clock speed and lengthened the design schedule.

The core of the design — the main processor — was termed the General Data Processor (GDP) and built as two integrated circuits: one (the 43201) to fetch and decode instructions, the other (the 43202) to execute them. Most systems would also include the 43203 Interface Processor (IP) which operated as a channel controller for I/O, and an Attached Processor (AP), a conventional Intel 8086 which provided "processing power in the I/O subsystem".[4]

These were some of the largest designs of the era. The two-chip GDP had a combined count of approximately 97,000 transistors while the single chip IP had approximately 49,000. By comparison, the Motorola 68000 (introduced in 1979) had approximately 40,000 transistors.

In 1983, Intel released two additional integrated circuits for the iAPX 432 Interconnect Architecture: the 43204 Bus Interface Unit (BIU) and 43205 Memory Control Unit (MCU). These chips allowed for nearly glueless multiprocessor systems with up to 63 nodes.

Some of the innovative features of the iAPX 432 were detrimental to good performance. In many cases, the iAPX 432 had a significantly slower instruction throughput than conventional microprocessors of the era, such as the National Semiconductor 32016, Motorola 68010 and Intel 80286. One problem was that the two-chip implementation of the GDP limited it to the speed of the motherboard's electrical wiring. A larger issue was the capability architecture needed large associative caches to run efficiently, but the chips had no room left for that. The instruction set also used bit-aligned variable-length instructions instead of the usual semi-fixed byte or word-aligned formats used in the majority of computer designs. Instruction decoding was therefore more complex than in other designs. Although this did not hamper performance in itself, it used additional transistors (mainly for a large barrel shifter) in a design that was already lacking space and transistors for caches, wider buses and other performance oriented features. In addition, the BIU was designed to support fault-tolerant systems, and in doing so up to 40% of the bus time was held up in wait states.

Another major problem was its immature and untuned Ada compiler. It used high-cost object-oriented instructions in every case, instead of the faster scalar instructions where it would have made sense to do so. For instance the iAPX 432 included a very expensive inter-module procedure call instruction, which the compiler used for all calls, despite the existence of much faster branch and link instructions. Another very slow call was enter_environment, which set up the memory protection. The compiler ran this for every single variable in the system, even when variables were used inside an existing environment and did not have to be checked. To make matters worse, data passed to and from procedures was always passed by value-return rather than by reference. When running the Dhrystone benchmark, parameter passing took ten times longer than all other computations combined.[14]

According to the New York Times, "the i432 ran 5 to 10 times more slowly than its competitor, the Motorola 68000".[15]

The iAPX 432 was one of the first systems to implement the new IEEE-754 Standard for Floating-Point Arithmetic.[16]

An outcome of the failure of the 432 was that microprocessor designers concluded that object support in the chip leads to a complex design that will invariably run slowly, and the 432 was often cited as a counter-example by proponents of RISC designs. However, some hold that the OO support was not the primary problem with the 432, and that the implementation shortcomings (especially in the compiler) mentioned above would have made any CPU design slow. Since the iAPX 432 there has been only one other attempt at a similar design, the Rekursiv processor, although the INMOS Transputer's process support was similar — and very fast.

Intel had spent considerable time, money, and mindshare on the 432, had a skilled team devoted to it, and was unwilling to abandon it entirely after its failure in the marketplace. A new architect—Glenford Myers—was brought in to produce an entirely new architecture and implementation for the core processor, which would be built in a joint Intel/Siemens project (later BiiN), resulting in the i960-series processors. The i960 RISC subset became popular for a time in the embedded processor market, but the high-end 960MC and the tagged-memory 960MX were marketed only for military applications.

According to the New York Times, Intel's collaboration with HP on the Merced processor (later known as Itanium) was the company's comeback attempt for the very high-end market.[15]

The iAPX 432 has hardware and microcode support for object-oriented programming and capability-based addressing.[18] The system uses segmented memory, with up to 2²⁴ segments of up to 64 KB each, providing a total virtual address space of 2⁴⁰ bytes. The physical address space is 2²⁴ bytes (16 MB).

Programs are not able to reference data or instructions by address; instead they must specify a segment and an offset within the segment. Segments are referenced by Access Descriptors (ADs), which provide an index into the system object table and a set of rights (capabilities) governing accesses to that segment. Segments may be "access segments", which can only contain Access Descriptors, or "data segments" which cannot contain ADs. The hardware and microcode rigidly enforce the distinction between data and access segments, and will not allow software to treat data as access descriptors, or vice versa.

System-defined objects consist of either a single access segment, or an access segment and a data segment. System-defined segments contain data or access descriptors for system-defined data at designated offsets, though the operating system or user software may extend these with additional data. Each system object has a type field which is checked by microcode, such that a Port Object cannot be used where a Carrier Object is needed. User programs can define new object types which will get the full benefit of the hardware type checking, through the use of Type Control Objects (TCO).

In Release 1 of the iAPX 432 architecture, a system-defined object typically consisted of an access segment, and optionally (depending on the object type) a data segment specified by an access descriptor at a fixed offset within the access segment.

By Release 3 of the architecture, in order to improve performance, access segments and data segments were combined into single segments of up to 128 kB, split into an access part and a data part of 0–64 KB each. This reduced the number of object table lookups dramatically, and doubled the maximum virtual address space.[19]

The iAPX432 recognizes fourteen types of predefined system objects:[20]^{:pp.1–11–1–12}

• instruction object contains executable instructions
• domain object represents a program module and contains references to subroutines and data
• context object represents the context of a process in execution
• type-definition object represents a software-defined object type
• type-control object represents type-specific privilege
• object table identifies the system's collection of active object descriptors
• storage resource object represents a free storage pool
• physical storage object identifies free storage blocks in memory
• storage claim object limits storage that may be allocated by all associated storage resource objects
• process object identifies a running process
• port object represents a port and message queue for interprocess communication
• carrier Carriers carry messages to and from ports
• processor contains state information for one processor in the system
• processor communication object is used for interprocessor communication

Software running on the 432 does not need to explicitly deallocate objects that are no longer needed. Instead, the microcode implements part of the marking portion of Edsger Dijkstra's on-the-fly parallel garbage collection algorithm (a mark-and-sweep style collector).[21] The entries in the system object table contain the bits used to mark each object as being white, black, or grey as needed by the collector. The iMAX 432 operating system includes the software portion of the garbage collector.[22]

Executable instructions are contained within a system "instruction object".[20]^:p.7–3 Since instructions are bit-aligned a 16-bit bit displacement into the instruction object allows the object to contain up to 8192 bytes of instructions (65,536 bits).

Instructions consist of an operator, consisting of a class and an opcode, and zero to three operand references. "The fields are organized to present information to the processor in the sequence required for decoding". More frequently used operators are encoded using fewer bits.[20]^:p.7–6 The instruction begins with the 4 or 6 bit class field which indicates the number of operands, called the order of the instruction, and the length of each operand. This is optionally followed by a 0 to 4 bit format field which describes the operands (if there are no operands the format is not present). Then come zero to three operands, as described by the format. The instruction is terminated by the 0 to 5 bit opcode, if any (some classes contain only one instruction and therefore have no opcode). "The Format field permits the GDP to appear to the programmer as a zero-, one-, two-, or three-address architecture." The format field indicates that an operand is a data reference, or the top or next-to-top element of the operand stack.[20]^{:pp.7–3–7–5}