What are the buses known as A, B, C in the following architecture of 8086? I cannot find any information about these.
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A-bus is the internal 16-bit ALU data bus.
C-Bus is the internal 20-bit address bus, 16-bit data bus, and possibly control lines of the BIU bus.
B-bus has no true name but the function of the adder ALU is to add the shifted 16-bits (Starting Address of 64 Kbyte segment) CS (Code Segment) to the 16-bits IP (Instruction Pointer - Offset into CS for next instruction) to get the 20-bit Physical Address. The right side of both diagrams allows the segment registers to be accessed. I'd guess B-bus's name would be Offset bus.
From my explanation, it's clear why these names are not common. They don't really correlate with the way the 8086 works. Assuming A-bus means Address can be dangerous.
I'd read the page where you got the graphic. It does explain the operation (as someone pointed out). If there is no reference to something that is in a diagram, it's usually because it's a bit more involved than a straight correlation.
According to this page, "A-Bus" is referring to "ALU Bus". C-Bus is referring to "Control logic BUS". The B-Bus looks like just a name..