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 requires 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 also saw use as a micro controller, by virtue of its low component count. Both designs were eclipsed for desktop computers by the compatible Zilog Z80, which took over most of the CP/M computer market as well as taking a share of the booming home computer market in the early-to-mid-1980s.
The 8085 had a 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 longer than the product life of desktop computers)
Description
The 8085 is a conventional von Newman design based on the Intel 8080. Unlike the 8080 it does not multiplex state signals onto the data bus, but the 8-bit data bus was instead multiplexed with the lower part of the 16-bit address bus to limit the number of pins to 40. Pin No. 40 is used for the power supply (+5v) and pin No. 20 for ground. Pin No. 39 is used as the hold pin. Pins No. 15 to No. 8 are generally used for address buses. 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. Only a 5 Volt supply is needed, like competing processors and unlike the 8080. The 8085 uses approximately 6,500transistors.
The 8085 incorporates the functions of the 8224 (clock generator) and the 8228 (system controller), increasing the level of integration. A downside compared to similar contemporary designs (such as the Z80) was the fact that the buses required demultiplexing; however, address latches in the Intel 8155, 8355, and 8755 memory chips allowed a direct interface, so an 8085 along with these chips was almost a complete system.
The 8085 has extensions to support new interrupts, with 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.
Like the 8080, the 8085 can accommodate slower memories through externally generated wait states (pin 35, READY), and has provisions for Direct Memory Access (DMA) using HOLD and HLDA signals (pins 39 and 38). An improvement over the 8080 was that the 8085 can itself drive a piezoelectric crystal directly connected to it, and a built in clock generator generates the internal high amplitude two-phase clock signals at half the crystal frequency (a 6.14 MHz crystal would yield a 3.07 MHz clock, for instance).
The 8085 is a binary compatible follow up on the 8080, using the same basic instruction set as the 8080. Only a few minor instructions were new to the 8085 above the 8080 set.
Applications
For the extensive use of 8085 in various applications, the microprocessor is provided with an instruction set which consists of various instructions such as MOV, ADD, SUB, JMP etc. These instructions are written in the form of a program which is used to perform various operations such as branching, addition, subtraction, bitwise logical and bit shift operations. More complex operations and other arithmetic operations must be implemented in software. For example, multiplication is implemented using a multiplication algorithm.
The 8085 processor was used in a few early personal computers, for example, the TRS-80 Model 100 line used a OKI manufactured 80C85 (MSM80C85ARS). The CMOS version 80C85 of the NMOS/HMOS 8085 processor has several manufacturers. Some manufacturers provide variants with additional functions such as additional instructions. 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 1990s and early 2000s, including CRRES, Polar, FAST, Cluster, HESSI, theSojourner Mars Rover 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.
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