Sunday, 21 June 2015
About 8085 microprocessor
The Intel 8085 ("eighty-eighty-five") is an 8-bit microprocessor produced by Intel and introduced in 1977.  It is software-binary compatible with the more-famous Intel 8080 with only a few minor instructions added. However, it required less support circuitry, allowing simpler and less expensive microcomputer systems to be built. The "5" in the part number highlighted the fact that the 8085 uses a single +5-Volt (V) power supply by using depletion mode transistors, rather than requiring the +5 V, −5 V and +12 V supplies needed by the 8080. This brought it up with the competing Z80,a popular 8080-derived CPU introduced the year before. These processors could be used in computers running the CP/M operating system. The 8085 was supplied in a 40-pin DIP package. To maximise the functions on the available pins, the 8085 used a multiplexed address/data bus. However, an 8085 circuit would require an 8-bit address latch so Intel manufactured several support chips with an address latch built in. These include the 8755, with an address latch, 2 KB of EPROM and 16 I/O pins, and the 8155 with 256 bytes of RAM, 22 I/O pins and a 14 bit programmable Timer/Counter. The multiplexed address/data bus reduced the number of PCB tracks between the 8085 and such memory and I/O chips. Both the 8080 and the 8085 were eclipsed by the Zilog Z80 for desktop computers, which took over most of the CP/M computer market as well as 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 VT102 video terminal in the late 1970s, it served for new production throughout the lifetime of those products. This was typically longer than the product life of desktop computers. The 8085 is a conventional von Neumann 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 is 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 (+5 V) 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 6,500 transistors.  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) is 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 is almost a complete system. The 8085 has extensions to support new interrupts, with three maskable vectored 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. Interrupts are enabled by the EI instruction and disabled by the DI instruction. 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 is 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. The processor has seven 8-bit registers accessible to the programmer, named A, B, C, D, E, H, and L, where A is the 8-bit accumulator and the other six can be used as independent byte-registers or as three 16-bit register pairs, BC, DE, and HL, depending on the particular instruction. Some instructions use HL as a (limited) 16-bit accumulator. As in the 8080, the contents of the memory address pointed to by HL could be accessed as pseudo register M. It also has a 16-bit program counter and a 16-bit stack pointer to memory (replacing the 8008's internal stack). Instructions such as PUSH PSW, POP PSW affected the Program Status Word (accumulator and flags). The accumulator stores the results of arithmetic and logical operations, and the flags register bits (sign, zero, auxiliary carry, parity, and carry flags) are set or cleared according to the results of these operations. Intel produced a series of development systems for the 8080 and 8085, known as the MDS-80 Microprocessor System. The original development system had an 8080 processor. Later 8085 and 8086 support was added including ICE (in-circuit emulators). It is a large and heavy desktop box, about a 20" cube (in the Intel corporate blue color) which included a CPU, monitor, and a single 8 inch floppy disk drive. Later an external box was available with two more floppy drives. It runs the ISIS operating system and can also operate an emulator pod and an external EPROM programmer. This unit uses the Multibus card cage which was intended just for the development system. A surprising number of spare card cages and processors were being sold, leading to the development of the Multibus as a separate product. The later iPDS is a portable unit, about 8" x16" x20", with a handle. It has a small green screen, a keyboard built into the top, a 5¼ inch floppy disk drive, and ran the ISIS-II operating system. It can also accept a second 8085 processor, allowing a limited form of multi-processor operation where both processors run simultaneously and independently. The screen and keyboard can be switched between them, allowing programs to be assembled on one processor (large programs took awhile) while files are edited in the other. It has a bubble memory option and various programming modules, including EPROM and Intel 8048 and 8051 programming modules which are plugged into the side, replacing stand-alone device programmers. 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. A lower cost SDK-85 System Design Kit board contains an 8085 CPU, 8355 ROM containing a debugging monitor program, 8155 RAM and 22 I/O, 8279 hex keypad and 8-digit 7-segment LED, TTY (Teletype) 20 mA current loop serial interface. Pads were available for one more 2Kx8 8755 EPROM and another 256 byte RAM 8155 I/O Timer/Counter could be optionally added. All data, control and address signals are available on dual pin headers and a large prototype area is provided. Application- 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 is used in a few early personal computers, for example, the TRS-80 Model 100 line used an 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,the Sojourner 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. Pro-Log Corp. put the 8085 and supporting hardware on an STD Bus format card containing CPU, RAM, sockets for ROM/EPROM, I/O and external bus interfaces. The included Instruction Set Reference Card uses entirely different mnemonics for the Intel 8085 CPU, as the product was a direct competitor to Intel's Multibus card offerings.