Intel 8085

The Intel 8085 is an 8-bit microprocessor introduced by Intel in 1977. 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 and the VT100 video terminal 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).

CPU architecture

The 8085 Architecture follows the von Neumann architecture, with a 16-bit address bus, and a 8-bit data bus. The 8085 incorporated all the features of the 8224 (clock generator) and the 8228 (system controller), increasing the level of system integration. The 8085 along with an 8156 RAM and 8355/8755 ROM/PROM constituted a complete system. The 8085 used a multiplexed Data Bus and required the 825X-5 support chips. The address was split between the 8-bit address bus and 8-bit data bus. The on-chip address latch of 8155/8355/8755 memory chips allowed a direct interface with the 8085. The processor was designed using NMOS circuitry and the later "H" versions were implemented in Intel's enhanced NMOS process called HMOS, originally developed for fast static RAM products. The 8085 used 6,500 transistors

Registers:

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.

Buses

* 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.