Intel 8085

Intel 8085

An Intel 8085AH processor.
General information
Launched	mid 1970s
Common manufacturer	Intel
Performance
Max. CPU clock rate	3.07 Mhz to 5 Mhz
Architecture and classification
Instruction set	pre x86
Physical specifications
Socket	40 pin DIP

The Intel 8085 was an 8-bit microprocessor made by Intel in the mid-1970s. It was binary compatible with the more-famous Intel 8080 but required less supporting hardware, thus allowing simpler and less expensive microcomputer systems to be built.

The "5" in the model number came from the fact that the 8085 required only a +5-volt (V) power supply rather than the +5V, -5V and +12V supplies the 8080 needed. Both processors were sometimes used in computers running the CP/M operating system, and the 8085 later saw use as a microcontroller (much by virtue of its component count reducing feature). Both designs were eclipsed for desktop computers by the compatible but more capable Zilog Z80, which took over most of the CP/M computer market as well as taking a large share of the booming home computer market in the early-to-mid-1980s. The 8085 had a very long life as a controller. Once designed into such products as the DECTape controller card in the late 1970s, it continued to serve for new production throughout the life span of those products (generally many times longer than the new manufacture lifespan of desktop computers).

The 8085 Architecture follows the von Neumann architecture, with a 16bit address bus, and a 8bit data bus. But it is actually based on harvard concept

The 8085 can access 2¹⁶ (= 65,536) individual 8-bit memory locations, or in other words, its address space is 64 KB. Unlike some other microprocessors of its era, it has a separate address space for up to 2⁸ (=256) I/O ports. It also has a built in register array which are usually labeled A(Accumulator), B, C, D, E, H, and L. Further special-purpose registers are the 16-bit Program Counter (PC), Stack Pointer (SP), and 8-bit flag register F. The microprocessor has three maskable interrupts (RST 7.5, RST 6.5 and RST 5.5), one Non-Maskable interrupt (TRAP), and one externally serviced interrupt (INTR). The RST n.5 interrupts refer to actual pins on the processor-a feature which permitted simple systems to avoid the cost of a separate interrupt controller chip.

• Address bus - 16 line bus accessing 2¹⁶ memory locations (64 KB) of memory.
• Data bus - 8 line bus accessing one 8-bit byte of data in one operation. Data bus width is the traditional measure of processor bit designations, as opposed to address bus width, resulting in the 8-bit microprocessor designation.
• Control buses - Carries the essential signals for various operations.

Intel produced a series of development systems for the 8080 and 8085, known as the Personal Development System. The original PDS was a large box (in the Intel corporate blue colour) which included a CPU and monitor, and used 8 inch floppy disks. It ran the ISIS operating system and could also operate an emulator pod and EPROM programmer. The later iPDS was a much more portable unit featuring a small green screen and a 5¼ inch floppy disk drive, and ran the ISIS-II operating system. It could also accept a second 8085 processor, allowing a limited form of multi-processor operation where both CPUs shared the screen, keyboard and floppy disk drive. In addition to an 8080/8085 assembler, Intel produced a number of compilers including PL/M-80 and Pascal languages, and a set of tools for linking and statically locating programs to enable them to be burnt into EPROMs and used in embedded systems.

The 8085 as designed was upward-compatible in instruction set to the 8080, but had extensions to support new hardware (principally the RST n.5 interrupts) and to provide more efficient code. The hardware support changes were announced and supported, but the software upgrades were not supported by the assembler, user manual or any other means. At times it was claimed they were not tested when that was false.

The 8085 can accommodate slower memories through externally generated Wait states (pin 35, READY), and also has provisions for Direct Memory Access (DMA) using HOLD and HLDA signals (pins 39 and 38).

The 8085 runs on a 6.14 MHz crystal, connected to X1 and X2 (pins 1 and 2). There is divide by 2 counter in the 8085 causing it to actually run at 3.07MHz. A higher speed selection from the same production lots runs at 5 MHz.

The 8085 processor has found marginal use in small scale computers up to the 21st century. The CMOS version 80C85 of the NMOS/HMOS 8085 processor has/had several manufacturers, and some versions (eg. Tundra Semiconductor Corporation's CA80C85B) have additional functionality, eg. extra machine code instructions. One niche application for the rad-hard version of the 8085 has been in on-board instrument data processors for several NASA and ESA space physics missions in the 1990's and early 2000's, including CRRES, Polar, FAST, Cluster, HESSI, and THEMIS. The Swiss company SAIA used the 8085 and the 8085-2 as the CPUs of their PCA1 line of programmable logic controllers during the 1980s.

In many engineering schools in Brazil, India, Germany, Greece and Malaysia, the 8085 processor is used in introductory microprocessor courses.

8085 Simulators exist aplenty for educational use. One such, is the GNUSim8085 [1], which works on GNU/Linux and Windows machines.