DEC's PDP-4 (1962) and PDP-11 (1970). As the I/O operations needed to cause a read operation on a minicomputer I/O device were typically different for different device controllers, different bootstrap programs were needed for different devices. DEC later added, in 1971, an optional diode matrix read-only memory for the PDP-11 that stored a bootstrap program of up to 32 words (64 bytes). It consisted of a printed circuit card, the M792, that plugged into the Unibus and held a 32 by 16 array of semiconductor diodes. With all 512 diodes in place, the memory contained all "one" bits; the card was programmed by cutting off each diode whose bit was to be "zero". DEC also sold versions of the card, the BM792-Yx series, pre-programmed for many standard input devices by simply omitting the unneeded diodes. Following the older approach, the earlier PDP-1 has a hardware loader, such that an operator need only push the "load" switch to instruct the paper tape reader to load a program directly into core memory. The PDP-7, PDP-9, and PDP-15 successors to the PDP-4 have an added Read-In button to read a program in from paper tape and jump to it. The Data General Supernova used front panel switches to cause the computer to automatically load instructions into memory from a device specified by the front panel's data switches, and then jump to loaded code. Early minicomputer boot loader examples In a minicomputer with a paper tape reader, the first program to run in the boot process, the boot loader, would read into core memory either the second-stage boot loader (often called a Binary Loader) that could read paper tape with checksum or the operating system from an outside storage medium. Pseudocode for the boot loader might be as simple as the following eight instructions: Set the P register to 9 Check paper tape reader ready If not ready, jump to 2 Read a byte from paper tape reader to accumulator Store accumulator to address in P register If end of tape, jump to 9 Increment the P register Jump to 2 A related example is based on a loader for a Nicolet Instrument Corporation minicomputer of the 1970s, using the paper tape reader-punch unit on a Teletype Model 33 ASR teleprinter. The bytes of its second-stage loader are read from paper tape in reverse order. Set the P register to 106 Check paper tape