Common Usage
An operator would stand at the front panel to bootstrap the computer, to debug running programs, and to find hardware faults. Typically, the operator would read from a scrap of paper containing a short series of bootstrap instructions that would be hand-entered using the toggle switches. First, the operator would set the "address" switch, and enter the address in binary using the switches. Then the operator would set the "value" switch, and then enter the value intended for that address. After punching in a dozen or so of these instructions (most computers had a "deposit next" button, which would deposit subsequent values in subsequent addresses, relieving the operator of needing to toggle in addresses), the operator would then set the starting address of the bootstrap program and press the "run" switch to begin the execution of the program. Often, the bootstrap would turn on the punched tape reader, which would load a somewhat longer program, which in turn would load the operating system from disk.
Some machines accelerated the bootstrap process by allowing the operator to set the switches to one or two machine language instructions and then directly executing those instructions. Other machines allowed I/O devices to be explicitly commanded from the front panel (for example, "Read-In Preset" on the PDP-10 or the accessing of memory-mapped I/O devices on a PDP-11). Some machines also contained various bootstrap programs in ROM and all that was required to boot the system was to start it executing at the address of the correct ROM program.
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