Intel 8085
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Image:Intel P8085AH-2.jpg 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 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 later eclipsed 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.
Buses:
- Address bus - 16 line bus accessing 216 memory locations (64KiB) 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 5 1/4 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 can access 216 (= 65,536) individual 8-bit memory locations, or in other words, its address space is 64k bytes. Unlike some other microprocessors of its era, it has a separate address space for up to 2^8 (=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 hardware based interrupt operations which are found in pins 7 through 9, these are called RST 7.5, RST 6.5, and RST 5.5 respectively. The 8085 has a TRAP interrupt which cannot be disabled (that is, TRAP is a Non-Maskable interrupt or NMI) and an INTR interrupt. Use of the INTR requires an external Programmable Interrupt Controller such as an 8259.
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 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.
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