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Design Considerations

The use of a Z80 microprocessor calls for a personal computer. However, LC-81 is not planned as such, it won't never have a keyboard, a video monitor and much less a mouse. Instead, it offers a traditional console made of switches and lamps that better resembles a DEC PDP11 than an Sinclair.

True minicomputer in the 70s were mostly used to control machinery and to analyze scientific data in laboratories among others, worth to mention that of function as network routers in the ARPANET.

LC-81 is much less powerful than those real machines but my plans as per applications are more in that direction.

From the operational point of view, there is no "built-in" software in the LC-81. On power-on or reset, the machine does not "boot", it rather goes to Halt mode and nothing else happens until the Operator operates the computer's front panel (Console) to enter or run Software from some place.

Software usually come from tape, but part of it resides in the Master Controller's EPROM, notably an Absolute Loader program that serves to bring software from external media into LC-81 memory. To start this or any other program, the Operator must enter its address in the Console, press the Load button, then the Start button. Software can also be entered into memory directly from the computer's Console, byte after byte, in binary.

The memory contained into the Master Controller Unit (MC) is said to be "private" because it is isolated from the EXT-BUS by the mean of 3rd-state buffers so no external device can gain access to it in any way.

Memory can be extended (up to 64KB total) by building an external Memory Unit. That memory would said to be "shared" because is accessible from both the CPU and DMA capable peripherals.

The use of a separate private and shared memory allows for concurrent operations between DMA operations and normal software execution as long as the later takes place in private memory.

The bus connecting all units in the LC-81 has only one line for interrupt request (IRQ) and one for interrupt acknowledge (IAK). Multiple nested interrupts are possible, however, because those lines are daisy-chained among units. The peripheral asserting the interrupt request opens the chain behind it to prevent others to interrupt, taking priority this way in the event of simultaneous interrupt requests.

This contrasts with having multiple IRQ lines in the bus and presents the advantage of simplicity in the bus at the expense of making things more complex in the peripheral design.

Storage technology for the LC-81 minicomputer must be "historically correct". Flash drive, for example, is not a good choice in that respect. Real legacy devices, however, such as paper punch tapes are out of my reach so I am considering intermediate solutions such as audio tapes, floppy drives and EPROM cartridges.

In general, I am more mentally oriented to tapes rather than disks. One reason is simplicity, the other is that working with tapes is interesting because of the limitations imposed by its sequential nature.

The terminology employed in the documentation of this project tries to reflect the historical context that LC-81 represents (1970s) to avoid confusion with terms found in modern systems such as UNIX and PCs.

For example, the operating system (currently under development) is referred to as a "Control Program", a term borrowed from IBM System/360's batch processing system; that is because mine is being designed for batch processing rather than time-sharing.