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Originally Posted by BookishDreamer
Not to me, but that's because I've never heard of them or have a clue when they were used.  |
Assembly Language is the lowest level language for a device, and all machines have a version of it. Each assembly language statement normally decodes to one machine language instruction. In higher level languages like C, a line of C code will compile into
many machine language instructions.
Early versions of the C language would compile to Assembler, which would then be assembled into actual machine code by the system assembler. It was possible to interrupt the compilation at the Assembler output stage, and then go in and optimize that before assembling it into the actual machine code you would run. That doesn't happen now either: compilers have been syteadily improved, and can generally do a better job of optimization than the programmer.
In the old days, programmers would write in Assembler when they needed to squeeze that last possible bit of performance from the machine. Assembler allows the tightest and most efficient code, but it's significantly harder to write and maintain. That is seldom done now, as hardware has gotten fast and cheap enough that it's usually more cost effective to get more hardware than to spend the time and effort required to get the code to run acceptably on what you have.
Assembler still sees usage in things like device drivers that talk directly to the hardware, and embedded applications in things like set top boxes and cell phones.
370 JCL is an IBM mainframe curiousity. IBM mainframes are batch processing machines, where jobs are submitted to be run in batches. The MVS operating system has a component called JES (Job Entry Subsystem), and JCL (Job Control Language) that you use to control it. All mainframe batch jobs are run from "procs" -- sets of JCL statements that define the job, what programs will be run as part of it, what data those programs will process and where it resides, and where to send the output.
I think I still have an OS 370 JCL manual around somewhere...
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Dennis