When I started my career as a field engineer for Honeywell mainframe computers in the late 1970s, I worked a lot of swing and midnight shifts. While day shift was always pretty busy, the night shifts were often boring. To entertain myself, I read the CPU manuals with the goal of being able to modify the diagnostic programs used to test the computers. Occasionally it proved handy to load one of the diagnostics and then to patch them in memory to loop through operations which were failing. This allowed using an oscilloscope to trace signals of interest though the 80 wire-wrap boards which made up the CPU.
Eventually writing these machine language programs became my favorite pastime on slow nights. Part of the draw was the maintenance panel switches which made it easy to read and write memory locations. There was a definite thrill to getting a program working and watching its progress via the flashing lights on the maintenance panel.
For those who aren't familiar with low level programming, machine language programming involves directly entering the binary encoded instructions into memory locations for later execution. More people are familiar with assembly language programming which replaced the binary programming with mnemonic names for the instructions and any modifiers. For example, a Honeywell mainframe had an instruction called LDA which loaded the A (or accumulator) register with some value. In machine language programming, that LDA instruction had the opcode of octal 235. Older mainframes often used octal encoding instead of the hexadecimal encoding which is more often used today. The other convenience offered by using assembly language over machine language is that the assembler would calculate the addresses automatically rather than forcing you to manually calculate the address offsets by hand which was painful.
My second job was as a field engineer for DEC PDP-11 minicomputers. These smaller machines were so much less complex than the mainframes that fixing the hardware wasn't much of a challenge. The saving grace was the PDP-11 instruction set was simple enough to allow me to quickly come up to speed on its machine language. When I was in Boston for training, I wrote a machine language program to determine which terminal connected to the PDP-11 had had data entered on its keyboard. Apparently the way I approached programming was different than most people's because the instructors had trouble figuring out how my program worked.
Believe it or not, the ability to decipher machine language is still useful when I have to use gdb to debug a program.
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