Intel 8085
 |
 An Intel 8085AH processor. | |
| General information | |
|---|---|
| Launched | 1977 |
| Discontinued | 1990s |
| Common manufacturer |
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| Performance | |
| Max. CPU clock rate | 3,5 and 6 MHz |
| Architecture and classification | |
| Instruction set | pre x86 |
| Physical specifications | |
| Package |
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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, 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 Neumann design based on the Intel 8080. Unlike the 8080 it had no state signals multiplexed 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. 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 was needed, like competing processors and unlike the 8080. The 8085 used approximately 6,500 transistors.[1]
The 8085 incorporated the functionality 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 had extensions to support new interrupts: It had 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 could accommodate slower memories through externally generated wait states (pin 35, READY), and had 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).
Programming model
The 8085 was a binary compatible follow up on the 8080, using the same basic instruction set as the 8008 (developed by Computer Terminal Corporation). Only a few minor instructions were new to the 8085 above the 8080 set.
Registers
The processor had seven 8-bit registers named A, B, C, D, E, H, and L, where A was the 8-bit accumulator and the other six could be used as independent byte-registers or as three 16-bit register pairs, BC, DE, and HL, depending on the particular instruction. Some instructions also enabled HL to be used as a (limited) 16-bit accumulator, and as in the 8080, the contents of the memory address pointed to by HL could be accessed as pseudoregister M. It also had a 16-bit stack pointer to memory (replacing the 8008's internal stack), and a 16-bit program counter.
Commands/instructions
As in many other 8-bit processors, all instructions were encoded in a single byte (including register-numbers, but excluding immediate data), for simplicity. Some of them were followed by one or two bytes of data, which could be an immediate operand, a memory address, or a port number. Like larger processors, it had automatic CALL and RET instructions for multi-level procedure calls and returns (which could even be conditionally executed, like jumps) and instructions to save and restore any 16-bit register-pair on the machine stack. There were also eight one-byte call instructions (RST) for subroutines located at the fixed addresses 00h, 08h, 10h,...,38h. These were intended to be supplied by external hardware in order to invoke a corresponding interrupt-service routine, but were also often employed as fast system calls. The most sophisticated command was XTHL, which was used for exchanging the register pair HL with the value stored at the address indicated by the stack pointer.
8-bit instructions
Most 8-bit operations could only be performed on the 8-bit accumulator (the A register). For two operand 8-bit operations, the other operand could be either an immediate value, another 8-bit register, or a memory cell addressed by the 16-bit register pair HL. Direct copying was supported between any two 8-bit registers and between any 8-bit register and a HL-addressed memory cell. Due to the regular encoding of the MOV-instruction (using a quarter of available opcode space) there were redundant codes to copy a register into itself (MOV B,B, for instance), which were of little use, except for delays. However, what would have been a copy from the HL-addressed cell into itself (i.e., MOV M,M) was instead used to encode the HLT instruction, halting execution until an external reset or interrupt occurred.
16-bit operations
Although the 8085 was generally an 8-bit processor, it also had limited abilities to perform 16-bit operations: Any of the three 16-bit register pairs (BC, DE, HL) or SP could be loaded with an immediate 16-bit value (using LXI), incremented or decremented (using INX and DCX), or added to HL (using DAD). LHLD loaded HL from directly-addressed memory and SHLD stored HL likewise. The XCHG operation exchanged the values of HL and DE. By adding HL to itself, it was possible to achieve the same result as a 16-bit arithmetical left shift with one instruction. The only 16 bit instruction that affected any flag was DAD (adding HL to BC, DE, HL or SP), which updated the carry flag to facilitate 24-bit or larger additions and left shifts (for a floating point mantissa for instance). Adding the stack pointer to HL was useful for indexing variables in (recursive) stack frames. It was possible to allocate stack frames (using DAD SP and SPHL) and to branch via a pointer computed in HL (using PCHL). These abilities made it feasible to compile languages such as PL/M, Pascal, or C with 16-bit variables and produce read-only 8085 machine code.
Subtraction and bitwise logical operations on 16 bits had to be done in 8-bit steps. Operations that had to be implemented by program code (subroutine libraries) included comparisons of signed integers as well as multiply and divide.
Input/output scheme
The 8085 supported up to 256 input/output (I/O) ports, accessed via dedicated I/O instructions—taking port addresses as operands. This I/O mapping scheme was regarded as an advantage, as it freed up the processor's limited address space. Many CPU architectures instead use a common address space without the need for dedicated I/O instructions, although a drawback in such designs may be that special hardware must be used to insert wait states as peripherals are often slower than memory. However, in some simple 8080 computers, I/O was indeed addressed as if they were memory cells, "memory mapped", leaving the I/O commands unused. I/O addressing could also sometimes employ the fact that the processor would output the same 8-bit port address to both the lower and the higher address byte (i.e. IN 05h would put the address 0505h on the 16-bit address bus). Similar I/O-port schemes are used in the 8080-compatible Zilog Z80 as well as the closely related x86 families of microprocessors.
Development system
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 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.[citation needed]
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 has found marginal use in small scale computers up to the 21st century. The TRS-80 Model 100 line uses a 80C85. 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 1990s and early 2000s, including CRRES, Polar, FAST, Cluster, HESSI, Sojourner (rover)[2], 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.
Pin diagram:- Pin #40 is used for power supply(+5v) and pin #20 for ground. Pin #39 is used for hold pin. Pins #15 to #8 are generally used for address buses.
MCS-85 Family
The 8085 CPU was only one part of a much larger family of chips developed by Intel, for building a complete system. Although the 8085 CPU itself was not a great success, many of these support chips (or their descendents) later found their use in combination with the 8086 microprocessor. For example, the original IBM PC based on the Intel 8088 processor used many of these chips, and the prominence of IBM PC compatibles is why many of these chips are still in use today, although not as the chips themselves, but with their equivalent functionality embedded into larger VLSI chips, namely the "Southbridge" chips of modern PCs.
- 8007-Ram controller
- 8085-CPU
- 8155-RAM+ 3 I/O Ports+Timer
- 8156-RAM+ 3 I/O Ports+Timer
- 8185-SRAM
- 8202-Dynamic RAM Controller
- 8203-Dynamic RAM Controller
- 8205-1 Of 8 Binary Decoder
- 8206-Error Detection & Correction Unit
- 8207-DRAM Controller
- 8210-TTL To MOS Shifter & High Voltage Clock Driver
- 8212-8 Bit I/O Port
- 8216-4 Bit Parallel Bidirectional Bus Driver
- 8218/8219-Bus Controller
- 8222-Dynamic RAM Refresh Controller
- 8226-4 Bit Parallel Bidirectional Bus Driver
- 8231-Arithmetic Processing Unit
- 8232-Floating Point Processor
- 8237-DMA Controller
- 8251-Communication Controller
- 8253-Programmable Interval Timer
- 8254-Programmable Interval Timer
- 8255-Programmable Peripheral Interface
- 8256-Multifunction Support Controller
- 8257-DMA Controller
- 8259-Programmable Interrupt Controller
- 8271-Programmable Floppy Disk Controller
- 8272-Single/Double Density Floppy Disk Controller
- 8273-Programmable HDLC/SDLC Protocol Controller
- 8274-Multi-Protocol Serial Controller
- 8275-CRT Controller
- 8276-Small System CRT Controller
- 8278-Programmable KeyBoard Interface
- 8279-KeyBoard/Display Controller
- 8282-8-bit Non-Inverting Latch with Output Buffer
- 8283-8-bit Inverting Latch with Output Buffer
- 8291-GPIB Talker/Listener
- 8292-GPIB Controller
- 8293-GPIB Transceiver
- 8294-Data Encryption/Decryption Unit+1 O/P Port
- 8295-Dot Matrix Printer Controller
- 8296-GPIB Transceiver
- 8297-GPIB Transceiver
- 8355-16,384-bit (2048 x 8) ROM with I/O
- 8604-4096-bit (512 x 8) PROM
- 8702-2K-bit (265 x 8 ) PROM
- 8755-EPROM+2 I/O Ports
Educational Use
In many engineering schools [2] the 8085 processor is used in introductory microprocessor courses.
Simulators
8085 simulators exist aplenty for educational use. A completely free 8085 simulator with examples is available at http://8085simulator.codeplex.com Freely available open source variants include GNUSim8085, j8085sim and GSim85[3] working on both, GNU/Linux and Windows.Windows only version with complete source code and example(s) including support for Intel SDK can be found hosted on Codeplex at 8085 Instruction set simulator Closed source freeware simulators for the Microsoft Win32 platform include Win85 [4] (which also emulates undocumented operations of the chip) and Sim8085.
References
- William Stallings Computer Organization and Architecture: Designing for Performance 8th Ed. Prentice Hall, 2009 ISBN 0136073735
- Abhishek Yadav Microprocessor 8085, 8086 Firewall Media, 2008 ISBN 8131803562
External links
Datasheets