[Pgubook-readers] Counters and register size

[Woolly Thinking]

The very simplest answer to register - cpu size is that it is the number of 
wires that carry the cpu instructions or data.  This is a slight over 
simplification but 16 bit cpu's basicaly use 16 wires, 32 bit cpu's use 32 
wires, etc  When you hear that a computer has 64 bit addressability it means 
that it has the wiring for 64 "on/off" states carried on the individual wires.  
These in turn mean that 64 on-off states provide the highest binary number that 
can be defined by that number of bits.  The  "word" originally came from the 
concept of one individual wiring state that transmitted a computer logical 
concept within a the limits of the cpu's wiring.  So a 16 bit computer's word 
was 16 bits.  The wiring of a computer corresponds in all respects to this word 
size.  Every register is 16 bits on a 16 bit computer.  

The counter is a register that holds the address of the next executable 
instruction.  This is a binary number that specifies an address in ram 
(addressability).  This counter is incremented by the next fetch cycle.  It can 
also be altered by programming in which the old value is saved (for later 
restoration) and a new value provided.  This is the basis for all branch,  
perform, and goto instructions in all languages.  Assembler does this but 
requires programmer effort.