Early Microprocessors and Their Characteristics

Key Insight

The microprocessor, a central processing unit on a single silicon chip, originated in 1971 with the Intel 4004, which contained about 2300 transistors. Nearly three decades later, microprocessors for home computers approached 10000000 transistors. Despite this dramatic increase in complexity, the fundamental function of the microprocessor remained unchanged. To understand its core operation, it's most insightful to examine the first 'ready-for-prime-time' microprocessors, as later chips often introduce complexities that can obscure the foundational principles.

These historically significant microprocessors appeared in 1974: the Intel 8080 in April and the Motorola 6800 in August. Other contemporary chips included the 4-bit Texas Instruments TMS 1000 and National Semiconductor's PACE, the first 16-bit microprocessor. The Intel 8080 was initially priced at $360, a playful nod to IBM's million-dollar System/360. The 8080 is an 8-bit microprocessor with approximately 6000 transistors, operating at a 2 MHz clock speed, and capable of addressing 64 kilobytes of memory. The 6800 contained about 4000 transistors and also addressed 64 KB of memory, initially running at 1 MHz, with later versions reaching 1.5 and 2 MHz by 1977. Both chips are now available for $1.95.

These devices are more accurately referred to as single-chip microprocessors rather than 'computers on a chip,' as a complete computer also requires RAM, input/output devices, and other binding chips. A microprocessor's function can be understood by examining its external interactions, treating it as a 'black box.' Both the 8080 and 6800 were 40-pin integrated circuits, typically 2 inches long, half an inch wide, and 1/8 inch thick. The actual silicon wafer inside these early 8-bit microprocessors was less than 1/4 inch square; the packaging served to protect the chip and provide access to its input/output points via the pins. Understanding the chip's input/output signals and its instruction set is key to grasping its capabilities.