Characteristics of Early PC System

            Before learning more about CP/M, now it is time to learn something about the hardware that was available in the period of CP/M.
            For the convenience, we will discuss the computers with a video monitor (CRT) (Yes, that means not all the computers had video output, but video monitor was one of the requirements for CP/M). As one might guess, early personal computers too had a motherboard. That motherboard had a microprocessor chip, RAM (Random Access Memory), some ROM (Read Only Memory) chips, and several IC’s who are responsible to for all these chips to work in harmony. Furthermore, motherboard used to have some empty slots to insert additional “expansion circuit boards”, which were, and still, are simply called as “cards”. These cards included a video controller card, which was used to connect a video monitor to the computer. Other expansion cards included additional RAM, floppy, and hard disk controllers. The standard devices for input/output were printer or video, or both, and keyboard. The keyboard was plugged directly to the motherboard, which already had a keyboard controller chip built in it.
            At this point, one might ask, what is it so different from today’s hardware? Everything is almost the same except the video output being optional? Well this is a fair doubt, and question. The answer is all these hardware had very limited capabilities. The main memory used to range from few bytes to several kilobytes. The latest microprocessors were 8-bit microprocessors.
            But yet, there were some advantages in this time. Like, the disk block size, and format was fixed for floppies and hard disks as well (though hard disks were used rarely.) This gave a standardised file system design, which was based on this standard disk format.
            And talking about the interrupt* handling, it was easy thing in those days. As there was only one application supposed to run at a time (no multi-tasking), there was no need to switch between applications. For the same reasons, CPU scheduling was not required in the OS. The interrupts were mainly used for handling the I/O devices.
            That’s all we needed to know about the early hardware, till now. The interrupts and CPU scheduling are two new terms here. At this time, we don’t need to know about CPU scheduling, but interrupts are going to prove kind of important in “I/O Management”. An “interrupt” is a mechanism which is used to signal the OS that some event has occurred which requires attention. The interrupt is signaled either by hardware or by software. Hardware may trigger an interrupt by sending a signal to the CPU, usually by the way of the system bus. Software my trigger an interrupt executing a special program, called as “system call”.