Encyclopedia
The
Intel 8080 was an early
microprocessor designed and manufactured by
Intel. The 8-bit
CPU was released in April 1974 running at 2 MHz, and is generally considered to be the first truly usable microprocessor CPU design.
Description
Programming model
The Intel 8080 was the successor to the
Intel 8008; this was due to its being assembly language source-compatible, since it used the same instruction set developed by Computer Terminal Corporation. The 8080's large 40 pin
DIP packaging permitted it to provide a 16-bit address bus and an 8-bit data bus, allowing easy access to 64 kilobytes of memory.
Registers
The processor had seven 8-bit registers, six of which could be combined into three 16-bit register pairs . It also had the 8-bit accumulator, the 16-bit stack pointer to memory , and a 16-bit program counter.
Commands
Most of the 8-bit operations were possible between the accumulator and either one of the registers or the memory cell, indexed by the 16-bit value of the register pair HL. Moving operations were supported between any two registers, or between any register and the HL-indexed memory cell. The command system also had strange commands to move a byte from a given register into the same register . These commands were seldom used, however, unless programmed delays were needed. The command to move from the HL-indexed memory cell into the same memory cell always halted the processor until the external reset or interrupt signals were received. Thus instead of MOV M, M this command was marked as HLT and used for this purpose, when required.
All processor commands were coded by one byte, but some of them were followed by one or two bytes of data, a memory address, or a port number. The register-to-register data-move commands were all coded by one byte, making up about a quarter of the commands in the processor-command system. The processor had 8 commands to call the subroutines located at the fixed addresses at the beginning or the address space . These commands were frequently used in the interrupt-handling or system-library 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.
16-bit operations
Despite the fact that the 8080 was generally an 8-bit processor, it was also able to increment or decrement any register pair , add the register pairs , switch HL with DE and perform the 16-bit arithmetical shift with one command. Hence some 16-bit operations were already possible.
Input/output scheme
Input output port space
The 8080 supported up to 256 input/output ports, accessed from programs via dedicated I/O instructions—each instruction taking an I/O port address as its operand. This scheme—using a separate I/O address space—is now less commonly used than memory mapping of I/O ports/devices. At the time of the 8080's launch, this I/O mapping scheme was seen as an advantage, as it freed up the processor's limited number of address pins for the memory address space. In most other CPU architectures, however, the mapping of I/O ports in a common address space both for memory and I/O, gave a simpler instruction set; no need for separate I/O instructions. The 8080-style I/O port scheme continued into the
Intel 8085 and
x86 families of microprocessors.
Stack space
One of the bits in the processor state word was indicating that the processor is accessing data from the stack. Using this signal, it was possible to implement the separate stack memory space. However this feature was seldom used.
Shared memory implementations
The 8080 has the shared control signals for reading and writing both to/from memory and I/O ports and in basic computers was frequently connected using the shared memory map, accessing ports as the memory cells. The specialised I/O commands were either not used or were used knowing that the processor clones the 8 bit port address to the higher address byte .
The internal state word
For the more complicated system, during one phase of its working loop the processor set its "internal state byte" on the data bus. This byte contains flags which indicate whether the memory or I/O port is accessed and whether it was necessary to handle an interrupt.
The interrupt system state was also output on a separate pin. For simple systems, where the interrupts were not used, it is possible to find cases where this pin is used as an additional single-bit output port .
Pin usage
The address bus had its own 16 pins, and the data bus had 8 pins that were possible to use without any multiplexing. Using the two additional pins , it was possible to assemble simple microprocessor devices very easily. Only the separate IO space, interrupts and DMA required additional chips to decode the processor pin signals. However the processor load capacity was limited, and even simple computers frequently contained the bus amplifiers.
The processor required three power sources and two non-interlacing high-amplitude synchronization signals. However at least the late Soviet version ??580??80? was able to work with the single +5 V power source, +12 V pin being connected to the same +5 V and -5 V pin - to the ground. The processor consumed about 1.3 watts of power.
The pin usage table was described in the chip accompanying documentation as following:
Pin number |
Signal |
Type |
Comment |
|---|
| 1 | A10 |
Output | Address bus 10 |
| 2 | GND |
- | Ground |
| 3 | D4 |
Bidirectional |
Bidirectional data bus. The processor also transiently sets here the "processor state", providing information that the processor is currently doing:
*D0 reading interrupt command. In response to the interrupt signal, the processor was reading and executing a single arbitrary command with this flag raised. Normally the supporting chips provided the subroutine call command , transferring control to the interrupt handling code.
*D1 reading
*D2 accessing stack
*D3 doing nothing, has been halted by the HLT command
*D4 writing data to the output port
*D5 reading the first byte of the executable command
*D6 reading data from the input port
*D7 reading data from memory
|
4 | D5 |
| 5 | D6 |
| 6 | D7 |
| 7 | D3 |
| 8 | D2 |
| 9 | D1 |
| 10 | D0 |
| 11 | -5 V |
- | The -5 V power supply. This must be the first power source connected and the last disconnected, otherwise the processor will be damaged. |
| 12 | R |
Input | Reset. The signal forces execution of commands, located at address 0000. The content of other processor registers is not modified. This is an inverting input |
| 13 | DMA |
Input | Direct memory access request. The processor is requested to switch the data and address bus to the high impedance state. |
| 14 | INT |
Input | Interrupt request |
| 15 | CLC2
| Input | The second phase of the clock generator signal |
| 16 | ACK INT |
Output | The processor had two commands for setting the 0 or 1 level on this pin. The pin normally was supposed to be used for the interrupt control. However in the simple computers it was sometimes used just as the single bit output port for various purposes. |
| 17 | RD |
Output | Read |
| 18 | WR |
Output | Write . This is the inverted output, the active level being logical zero. |
| 19 | S |
Output | The active level indicates that the processor has set the "state word" on the data bus. The various bits of this state word provided the additional information for supporting the separate address and memory spaces, interrupts and direct memory access. This signal required to pass through additional logic before it could be used to write the processor state word from the data bus into some register. |
| 20 | 5 V | - | The + 5 V power supply |
|
| 21 | ACK DMA |
Output | Direct memory access confirmation. The processor switches data and address pins into the high impedance state, allowing other device to manipulate the bus |
| 22 | CLC1 |
Input | The first phase of the clock generator signal |
| 23 | RDY |
Input | Wait. With this signal it was possible to suspend processor's work. It was also used to support the hardware-based step-by step debugging mode. |
| 24 | WAIT |
Output | Wait |
| 25 | A0 |
Output |
Address bus |
| 26 | A1 |
| 27 | A2 |
| 28 | 12 V |
- | The +12 V power supply. This must be the last connected and first disconnected power source. |
| 29 | A3 |
Output |
The address bus, can switch into high impedance state on demand |
| 30 | A4 |
| 31 | A5 |
| 32 | A6 |
| 33 | A7 |
| 34 | A8 |
| 35 | A9 |
| 36 | A15 |
| 37 | A12 |
| 38 | A13 |
| 39 | A14 |
| 40 | A11 |
Literature, used for this table:
Physical implementation
The 8080
integrated circuit was manufactured in a
NMOS process using a minimum feature size of 6 µm. A single layer of metal was used to interconnect the approximately 6000 transistors in the design . The
die size was approximately 20 mm².
The industrial impact
Applications and successors
The 8080 was used in many early microcomputers, such as the
MITS Altair 8800 and
IMSAI 8080, forming the basis for machines running the
CP/M operating system . The first single-board microcomputer was based on the 8080. The company Landis & Gyr used it on its electrical metering data aquisition equipment, the Datagyr FAB during the early eighties.
Shortly after the launch of the 8080, the
Motorola 6800 competing design was introduced, and after that, the
MOS Technology 6502 variation of the 6800.
Zilog introduced the
Z80, which had a compatible machine-language instruction set and initially used the same assembly language as the 8080, but for legal reasons, Zilog developed a syntactically-different alternative assembly language for the Z80. At Intel, the 8080 was followed by the compatible and electrically more elegant
8085, and later by the assembly language compatible 16-bit
8086 and then the 8/16-bit
8088, which was selected by
IBM for its new
PC to be launched in 1981. The 8080, via its ISA, thus made a lasting impact on computer history.
The Soviet Union manufactured the complete 8080 analog KP580?K80 , where even pins were placed identically. This processor was the base of the Radio86RK , probably the most popular amateur single-board computer in the Soviet Union. Predecessor of Radio86RK was Micro-80 , and successor - Orion-128 that has graphical display, both was built on KP580 processor. According to some sources, the Soviet analog had two undocumented instructions, specific to itself; however, these were not widely known.
Industry change
The 8080 also changed how computers were created. When the 8080 was introduced, computer systems were usually created by computer manufacturers such as
Digital Equipment Corporation,
Hewlett Packard, or
IBM. A manufacturer would produce the entire computer, including processor, terminals, and system software such as compilers and operating system. The 8080 was actually designed for just about any application
except a complete computer system. Hewlett Packard developed the
HP 2640 series of smart terminals around the 8080. The HP 2647 was a terminal which ran
BASIC on the 8080.
Microsoft would create the first popular programming language for the 8080, and would later acquire
DOS for the
IBM-PC.
As the 8080 evolved into the largely compatible x86 family, PC's evolved into workstations and servers of 32 and 64 bits, with advanced memory protection, segmentation, and multiprocessing features, blurring the difference between small and large computers. The size of chips has grown so that the size and power of large x86 chips is not much different from high end architecture chips, and a common strategy to produce a very large computer is to network many x86 processors.
The basic architecture of the 8080 and its successors has replaced many proprietary midrange and mainframe computers, and withstood challenges of technologies such as RISC. Most computer manufacturers have abandoned producing their own processors below the highest performance points. Though x86 may not be the most elegant, or theoretically most efficient design, the sheer market force of so many dollars going into refining a design has made the x86 family today, and will remain for some time, the dominant processor architecture, even bypassing Intel's attempts to replace it with incompatible architectures such as the iAPX 432 and
Itanium.
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