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Inside the Apple-1's unusual MOS clock driver chip

Apple’s first product was the Apple-1 computer, introduced in 1976. This early microcomputer used an unusual type of storage for its display: shift register memory. Instead of storing data in RAM (random-access memory), it was stored in a 1024-position shift register. You put a bit into the shift register and 1024 clock cycles later, the bit pops out the other end. Since a shift-register memory didn’t require addressing circuitry, it could be manufactured more cheaply than a random-access memory chip.1 The downside, of course, is that you had to use bits as they became available, rather than access arbitrary memory locations. The behavior of shift-register memory was a good match for video circuitry, though, since characters are displayed on the screen in a fixed, repeating order (left to right and top to bottom).

Shift-register memory chips required clock pulses with high current and unusual voltages: from +5 volts to -11 volts. These pulses were provided by a special chip, the DS0025 Two-Phase MOS Clock Driver. This chip, introduced in 1969, was the first monolithic (i.e. integrated circuit) clock driver. In this blog post, I look inside the chip and explain how it was implemented.”

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