Intel 8086

Details

Buses and operation

All internal registers, as well as internal and external data buses, are 16 bits wide, which firmly established the "16-bit microprocessor" identity of the 8086. A 20-bit external address bus provides a 1 MiB physical address space (2²⁰ = 1,048,576 x 1 byte). This address space is addressed by means of internal memory "segmentation". The data bus is multiplexed with the address bus in order to fit all of the control lines into a standard 40-pin dual in-line package. It provides a 16-bit I/O address bus, supporting 64 KB of separate I/O space. The maximum linear address space is limited to 64 KB, simply because internal address/index registers are only 16 bits wide. Programming over 64 KB memory boundaries involves adjusting the segment registers (see below); this difficulty existed until the 80386 architecture introduced wider (32-bit) registers (the memory management hardware in the 80286 did not help in this regard, as its registers are still only 16 bits wide).

Hardware modes of 8086

Some of the control pins, which carry essential signals for all external operations, have more than one function depending upon whether the device is operated in min or max mode. The former mode is intended for small single-processor systems, while the latter is for medium or large systems using more than one processor (a kind of multiprocessor mode). Maximum mode is required when using an 8087 or 8089 coprocessor. The voltage on pin 33 (MN/MX) determines the mode. Changing the state of pin 33 changes the function of certain other pins, most of which have to do with how the CPU handles the (local) bus. The mode is usually hardwired into the circuit and therefore cannot be changed by software. The workings of these modes are described in terms of timing diagrams in Intel datasheets and manuals. In minimum mode, all control signals are generated by the 8086 itself.

Registers and instruction

The 8086 has eight more-or-less general 16-bit registers (including the stack pointer but excluding the instruction pointer, flag register and segment registers). Four of them, AX, BX, CX, DX, can also be accessed as 8-bit register pairs (see figure) while the other four, SI, DI, BP, SP, are 16-bit only.

Due to a compact encoding inspired by 8-bit processors, most instructions are one-address or two-address operations, which means that the result is stored in one of the operands. At most one of the operands can be in memory, but this memory operand can also be the destination, while the other operand, the source, can be either register or immediate. A single memory location can also often be used as both source and destination which, among other factors, further contributes to a code density comparable to (and often better than) most eight-bit machines at the time.

The degree of generality of most registers is much greater than in the 8080 or 8085. However, 8086 registers were more specialized than in most contemporary minicomputers and are also used implicitly by some instructions. While perfectly sensible for the assembly programmer, this makes register allocation for compilers more complicated compared to more orthogonal 16-bit and 32-bit processors of the time such as the PDP-11, VAX, 68000, 32016, etc. On the other hand, being more regular than the rather minimalistic but ubiquitous 8-bit microprocessors such as the 6502, 6800, 6809, 8085, MCS-48, 8051, and other contemporary accumulator-based machines, it is significantly easier to construct an efficient code generator for the 8086 architecture.

Another factor for this is that the 8086 also introduced some new instructions (not present in the 8080 and 8085) to better support stack-based high-level programming languages such as Pascal and PL/M; some of the more useful instructions are push mem-op, and ret size, supporting the "Pascal calling convention" directly. (Several others, such as push immed and enter, were added in the subsequent 80186, 80286, and 80386 processors.)

A 64 KB (one segment) stack growing towards lower addresses is supported in hardware; 16-bit words are pushed onto the stack, and the top of the stack is pointed to by SS:SP. There are 256 interrupts, which can be invoked by both hardware and software. The interrupts can cascade, using the stack to store the return addresses.

The 8086 has 64 K of 8-bit (or alternatively 32 K of 16-bit word) I/O port space.

Flags

The 8086 has a 16-bit flags register. Nine of these condition code flags are active, and indicate the current state of the processor: Carry flag (CF), Parity flag (PF), Auxiliary carry flag (AF), Zero flag (ZF), Sign flag (SF), Trap flag (TF), Interrupt flag (IF), Direction flag (DF), and Overflow flag (OF). Also referred to as the status word, the layout of the flags register is as follows:

Segmentation

There are also four 16-bit segment registers (see figure) that allow the 8086 CPU to access one megabyte of memory in an unusual way. Rather than concatenating the segment register with the address register, as in most processors whose address space exceeds their register size, the 8086 shifts the 16-bit segment four bits left before adding it to the 16-bit offset (16×segment + offset), therefore producing a 20-bit external (or effective or physical) address from the 32-bit segment:offset pair. As a result, any external address could be referred to by up to 2¹² = 4096 different segment:offset pairs.

Although considered complicated and cumbersome by many programmers, this scheme also has advantages; a small program (less than 64 KB) can be loaded starting at a fixed offset (such as 0000) in its own segment, avoiding the need for relocation, with at most 15 bytes of alignment waste.

Compilers for the 8086 family commonly support two types of pointer, near and far. Near pointers are 16-bit offsets implicitly associated with the program's code or data segment and so can be used only within parts of a program small enough to fit in one segment. Far pointers are 32-bit segment:offset pairs resolving to 20-bit external addresses. Some compilers also support huge pointers, which are like far pointers except that pointer arithmetic on a huge pointer treats it as a linear 20-bit pointer, while pointer arithmetic on a far pointer wraps around within its 16-bit offset without touching the segment part of the address.

To avoid the need to specify near and far on numerous pointers, data structures, and functions, compilers also support "memory models" which specify default pointer sizes. The tiny (max 64K), small (max 128K), compact (data > 64K), medium (code > 64K), large (code,data > 64K), and huge (individual arrays > 64K) models cover practical combinations of near, far, and huge pointers for code and data. The tiny model means that code and data are shared in a single segment, just as in most 8-bit based processors, and can be used to build .com files for instance. Precompiled libraries often come in several versions compiled for different memory models.

According to Morse et al.,. the designers actually contemplated using an 8-bit shift (instead of 4-bit), in order to create a 16 MB physical address space. However, as this would have forced segments to begin on 256-byte boundaries, and 1 MB was considered very large for a microprocessor around 1976, the idea was dismissed. Also, there were not enough pins available on a low cost 40-pin package for the additional four address bus pins.

In principle, the address space of the x86 series could have been extended in later processors by increasing the shift value, as long as applications obtained their segments from the operating system and did not make assumptions about the equivalence of different segment:offset pairs. In practice the use of "huge" pointers and similar mechanisms was widespread and the flat 32-bit addressing made possible with the 32-bit offset registers in the 80386 eventually extended the limited addressing range in a more general way.

The instruction stream is fetched from memory as words and is addressed internally by the processor to the byte level as necessary. An instruction stream queuing mechanism allows up to 6 bytes of the instruction stream to be queued while waiting for decoding and execution. The queue acts as a first-in-first-out (FIFO) buffer, from which the Execution Unit (EU) extracts instruction bytes as required. Whenever there is space for at least two bytes in the queue, the BIU will attempt a word fetch memory cycle. If the queue is empty (following a branch instruction, for example), the first byte into the queue immediately becomes available to the EU.

Porting older software

Small programs could ignore the segmentation and just use plain 16-bit addressing. This allows 8-bit software to be easily ported to the 8086. MS-DOS took advantage of this by providing an Application Programming Interface very similar to CP/M as well as including the simple .com executable file format, identical to CP/M. This was important when the 8086 and MS-DOS were new, because it allowed many existing CP/M (and other) applications to be quickly made available, greatly easing acceptance of the new platform.

Interrupts

Interrupts on the 8086 can be either software or hardware-initiated. Interrupts are long calls that also save the processor status. Interrupt routines typically end with a IRET instruction. All interrupts have a 8-bit interrupt number associated with them. This number is used to look up a segment:offset in a 256 element interrupt vector table stored at addresses 0-3FFH. When any type of interrupt is encountered, the processor status is pushed, CS and IP are pushed, and the interrupt number is multiplied by four to index a new execution address which is loaded from the vector table.

There are three types of software interrupt instructions: INT n, INTO, and a single-byte INT 3 used for debugging.

There are two kinds of hardware interrupts: maskable and non-maskable.

Non-maskable interrupts are higher priority than maskable interrupts. They cannot be disabled by interrupt enable. A low to high transition on the NMI pin essentially causes an INT 2 to execute.

Maskable interrupts are enabled and disabled by the STI and CLI instructions respectively. When the INTR is asserted by a hardware device, the 8086 asserts INTA twice, reading an 8-bit interrupt number from the bus. This number is multiplied by four to point to the associated interrupt service routine address in the vector table. Maskable interrupts are disabled when INTA is asserted, but are re-enabled upon executing the IRET instruction at the end of the interrupt service routine.

Example code

The following 8086 assembly source code is for a subroutine named _strtolower that copies a null-terminated ASCIIZ character string from one location to another, converting all alphabetic characters to lower case. The string is copied one byte (8-bit character) at a time.

The example code uses the BP (base pointer) register to establish a call frame, an area on the stack that contains all of the parameters and local variables for the execution of the subroutine. This kind of calling convention supports reentrant and recursive code and has been used by Algol-like languages since the late 1950s. A flat memory model is assumed, specifically, that the DS and ES segments address the same region of memory.

Performance

Although partly shadowed by other design choices in this particular chip, the multiplexed address and data buses limit performance slightly; transfers of 16-bit or 8-bit quantities are done in a four-clock memory access cycle, which is faster on 16-bit, although slower on 8-bit quantities, compared to many contemporary 8-bit based CPUs. As instructions vary from one to six bytes, fetch and execution are made concurrent and decoupled into separate units (as it remains in today's x86 processors): The bus interface unit feeds the instruction stream to the execution unit through a 6-byte prefetch queue (a form of loosely coupled pipelining), speeding up operations on registers and immediates, while memory operations became slower (four years later, this performance problem was fixed with the 80186 and 80286). However, the full (instead of partial) 16-bit architecture with a full-width ALU meant that 16-bit arithmetic instructions could now be performed with a single ALU cycle (instead of two, via internal carry, as in the 8080 and 8085), speeding up such instructions considerably. Combined with orthogonalizations of operations versus operand types and addressing modes, as well as other enhancements, this made the performance gain over the 8080 or 8085 fairly significant, despite cases where the older chips may be faster (see below).

• EA = time to compute effective address, ranging from 5 to 12 cycles.
• Timings are best case, depending on prefetch status, instruction alignment, and other factors.

As can be seen from these tables, operations on registers and immediates were fast (between 2 and 4 cycles), while memory-operand instructions and jumps were quite slow; jumps took more cycles than on the simple 8080 and 8085, and the 8088 (used in the IBM PC) was additionally hampered by its narrower bus. The reasons why most memory related instructions were slow were threefold:

• Loosely coupled fetch and execution units are efficient for instruction prefetch, but not for jumps and random data access (without special measures).
• No dedicated address calculation adder was afforded; the microcode routines had to use the main ALU for this (although there was a dedicated segment + offset adder).
• The address and data buses were multiplexed, forcing a slightly longer (33~50%) bus cycle than in typical contemporary 8-bit processors.

However, memory access performance was drastically enhanced with Intel's next generation of 8086 family CPUs. The 80186 and 80286 both had dedicated address calculation hardware, saving many cycles, and the 80286 also had separate (non-multiplexed) address and data buses.

Floating point

The 8086/8088 could be connected to a mathematical coprocessor to add hardware/microcode-based floating-point performance. The Intel 8087 was the standard math coprocessor for the 8086 and 8088, operating on 80-bit numbers. Manufacturers like Cyrix (8087-compatible) and Weitek (not 8087-compatible) eventually came up with high-performance floating-point coprocessors that competed with the 8087.

Chip versions

The clock frequency was originally limited to 5 MHz, but the last versions in HMOS were specified for 10 MHz. HMOS-III and CMOS versions were manufactured for a long time (at least a while into the 1990s) for embedded systems, although its successor, the 80186/80188 (which includes some on-chip peripherals), has been more popular for embedded use.

The 80C86, the CMOS version of the 8086, was used in many portable computers and embedded systems, including the GridPad, Toshiba T1200, HP 110, and finally the 1998–1999 Lunar Prospector.

For the packaging, the Intel 8086 was available both in ceramic and plastic DIP packages.

List of Intel 8086

Derivatives and clones

Compatible—and, in many cases, enhanced—versions were manufactured by Fujitsu, Harris/Intersil, OKI, Siemens, Texas Instruments, NEC, Mitsubishi, and AMD. For example, the NEC V20 and NEC V30 pair were hardware-compatible with the 8088 and 8086 even though NEC made original Intel clones μPD8088D and μPD8086D respectively, but incorporated the instruction set of the 80186 along with some (but not all) of the 80186 speed enhancements, providing a drop-in capability to upgrade both instruction set and processing speed without manufacturers having to modify their designs. Such relatively simple and low-power 8086-compatible processors in CMOS are still used in embedded systems.

The electronics industry of the Soviet Union was able to clone the 8086 through both industrial espionage and reverse engineering. The resulting chip, K1810VM86, was binary and pin-compatible with the 8086.

i8086 and i8088 were respectively the cores of the Soviet-made PC-compatible EC1831 and EC1832 desktops. (EC1831 is the EC identification of IZOT 1036C and EC1832 is the EC identification of IZOT 1037C, developed and manufactured in Bulgaria. EC stands for Единая Система.) However, the EC1831 computer (IZOT 1036C) had significant hardware differences from the IBM PC prototype. The EC1831 was the first PC-compatible computer with dynamic bus sizing (US Pat. No 4,831,514). Later some of the EC1831 principles were adopted in PS/2 (US Pat. No 5,548,786) and some other machines (UK Patent Application, Publication No. GB-A-2211325, Published June 28, 1989).

Support chips

• Intel 8237: direct memory access (DMA) controller
• Intel 8251: universal synchronous/asynchronous receiver/transmitter at 19.2 kbit/s
• Intel 8253: programmable interval timer, 3x 16-bit max 10 MHz
• Intel 8255: programmable peripheral interface, 3x 8-bit I/O pins used for printer connection etc.
• Intel 8259: programmable interrupt controller
• Intel 8279: keyboard/display controller, scans a keyboard matrix and display matrix like 7-seg
• Intel 8282/8283: 8-bit latch
• Intel 8284: clock generator
• Intel 8286/8287: bidirectional 8-bit driver. In 1980 both Intel I8286/I8287 (industrial grade) version were available for US$16.25 in quantities of 100.
• Intel 8288: bus controller
• Intel 8289: bus arbiter
• NEC μPD765 or Intel 8272A: floppy controller

Microcomputers using the 8086

• The Intel Multibus-compatible single-board computer ISBC 86/12 was announced in 1978.
• The Xerox NoteTaker was one of the earliest portable computer designs in 1978 and used three 8086 chips (as CPU, graphics processor, and I/O processor), but never entered commercial production.
• Seattle Computer Products shipped S-100 bus based 8086 systems (SCP200B) as early as November 1979.
• The Norwegian Mycron 2000, introduced in 1980s
• The first Compaq Deskpro used an 8086 running at 7.16 MHz, but was compatible with add-in cards designed for the 4.77 MHz IBM PC XT and could switch the CPU down to the lower speed (which also switched in a memory bus buffer to simulate the 8088's slower access) to avoid software timing issues.
• An 8 MHz 8086-2 was used in the AT&T 6300 PC (built by Olivetti, and known globally under several brands and model numbers), an IBM PC-compatible desktop microcomputer. The M24 / PC 6300 has IBM PC/XT compatible 8-bit expansion slots, but some of them have a proprietary extension providing the full 16-bit data bus of the 8086 CPU (similar in concept to the 16-bit slots of the IBM PC AT, but different in the design details, and physically incompatible), and all system peripherals including the onboard video system also enjoy 16-bit data transfers. The later Olivetti M24SP featured an 8086-2 running at the full maximum 10 MHz.
• The IBM PS/2 models 25 and 30 were built with an 8 MHz 8086.
• The Amstrad PC1512, PC1640, PC2086, PC3086 and PC5086 all used 8086 CPUs at 8 MHz.
• The NEC PC-9801.
• The Tandy 1000 SL-series and RL machines used 9.47 MHz 8086 CPUs.
• The IBM Displaywriter word processing machine and the Wang Professional Computer, manufactured by Wang Laboratories, also used the 8086.
• NASA used original 8086 CPUs on equipment for ground-based maintenance of the Space Shuttle Discovery until the end of the space shuttle program in 2011. This decision was made to prevent software regression that might result from upgrading or from switching to imperfect clones.
• KAMAN Process and Area Radiation Monitors
• The Tektronix 4170 ran CP/M-86 and used an 8086 4170 Local Graphics Processing Unit Instruction Manual (PDF)

One of the most influential microcomputers of all, the IBM PC, used the Intel 8088, a version of the 8086 with an 8-bit data bus.