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steve1000
 Post subject: TTL 6502
PostPosted: Tue Apr 04, 2017 9:55 am 
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Joined: Tue Apr 04, 2017 9:23 am
Posts: 1
I have decided to build a cpu in ttl and basing the design on the 6502 micro.
I have read the datasheet and it says the ALU is not used to inc/dec the program counter!
Does this mean there is separate ALU dedicated to operations on the PC.
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Arlet
 Post subject: Re: TTL 6502
PostPosted: Tue Apr 04, 2017 10:15 am 
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Joined: Tue Nov 16, 2010 8:00 am
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Location: Gouda, The Netherlands
I wouldn't call it a real ALU. The PC can increment and load absolute addresses. Relative offsets (branches) are calculated by the ALU.
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BigEd
 Post subject: Re: TTL 6502
PostPosted: Tue Apr 04, 2017 10:36 am 
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Joined: Thu Dec 11, 2008 1:28 pm
Posts: 10985
Location: England
Indeed - the ALU is limited to 8 bits, which is why some operations have to take an extra cycle when there's a carry. For the PC incrementer, it must be able to operate across the full 16 bits in a single cycle. As it happens, incrementing is simpler than general arithmetic, and it has a couple of lookahead gates too to speed up the circuit.

Edit: see also this previous discussion
viewtopic.php?p=41205#p41205
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