Intel 8080 manual

Since their inception, digital computers have contin- uously become more efficient, expanding into new appli- cations with each major technological improvement. The advent of minicomputers enabled the inclusion of digital computers as a permanent part of various process control systems. Unfortunately, the size and cost of minicomputers in "dedicated" applications has limited their use. Another approach has been the use of custom built systems made up of "random logic" (i .e., logic gates, flip-flops, counters, etc.). However, the huge expense and development time involved in the design and debugging of these systems has restricted their use to large volume applications where the develop- ment costs could be spread over a large number of machines.

Today, Intel offers the systems designer a new alter- native ... the microcomputer. Utilizing the technologies and experience gained in becoming the world's largest supplier of LSI memory components, Intel has made the power of the digital computer available at the integrated circuit level. Using the n-channel silicon gate MOS process, Intel engi- neers have implemented the fast (2/ls. cycle) and powerful (72 basic instructions) 8080 microprocessor on a single LSI chip. When this processor is combined with memory and I/O circuits, the computer is complete. Intel offers a variety of random-access memory (RAM), read-only memory (ROM) and shift register circu~ts, that combine with the 8080 pro- cessor to form the MCS-80 microcomputer system, a system that can directly address and retrieve as many as 65,536 bytes stored in the memory devices.

The 8080 processor is packaged in a 40-pin dual in-line package (DIP) that allows for remarkably easy interfacing. The 8080 has a 16-bit address bus, a 8-bit bidirectional data bus and fully decoded, TTL-compatible control outputs. In addition to supporting up to 64K bytes of mixed RAM and ROM memory, the 8080 can address up to 256 input ports and 256 output ports; thus allowing for virtually unlimited system expansion. The 8080 instruction set includes con- ditional branching, decimal as well as binary arithmetic,

logical, register-to-register, stack control and memory refer- ence instructions. In fact, the 8080 instruction set is power- ful enough to rival the performance of many of the much higher priced minicomputers, yet the 8080 is upward soft- ware compatible with Intel's earlier 8008 microprocessor (Le., programs written for the 8008 can be assembled and executed on the 8080).

In addition to an extensive instruction set oriented to problem solving, the 8080 has another significant feature- SPEED. In contrast to random logic designs which tend to work in parallel, the microcomputer works by sequentially executing its program. As a result of this sequential execu- tion, the number of tasks a microcomputer can undertake in a given period of time is directly proportional to the execution speed of the microcomputer. The speed of exe- cution is the limiting factor of the realm of applications of the microcomputer. The 8080, with instruction times as short as 2 /lsec., is an order of magnitude faster than earlier generations of microcomputers, and therefore has an ex- panded field of potential applications.

The architecture of the 8080 also shows a significant improvement over earlier microcomputer designs. The 8080 contains a 16-bit stack pointer that controls the addressing of an external stack located in memory. The pointer can be initialized via the proper instructions such that any portion of external memory can be used as a last in/first out stack; thus enabling almost unlimited subroutine nesting. The stack pointer allows the contents of the program counter, the ac- cumulator, the condition flags or any of the data registers to be stored in or retrieved from the external stack. In addi- tion, multi-level interrupt processing is possible using the 8080's stack control instructions. The status of the pro- cessor can be "pushed" onto the stack when an interrupt is accepted, then "popped" off the stack after the interrupt has been serviced. This ability to save the contents of the pro- cessor's registers is possible even if an interrupt service routine, itself, is interrupted.