Machine code, in binary, using a row of switches on the front panel.
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I played with one of these as an undergraduate in the early 1970s. The main I/O device was a teletype terminal with a paper tape reader / puncher. I normally wrote assembly language using the keyboard. When I wanted to save something, either source code or an assembled program, I punched a paper tape.
The operating system was on a paper tape that I had to load after turning the power on. In order to load that paper tape, I first had to enter a short paper tape reading program via the front panel switches. About twenty 12-bit binary instructions, one at a time via the switch register, from a sheet of paper that we kept handy.