Re: How complex is the 6502?

[j0p7771@TAMUTS.TAMU.EDU (Jason Perez)]

In article <CGHLDo.EGG@actrix.gen.nz>,
David Empson <dempson@swell.actrix.gen.nz> wrote:
-In article <jpenne.753269520@ee.ualberta.ca> jpenne@ee.ualberta.ca 
-   (Jerry Penner) writes:
-- the operand before it feeds them to the adder.  So that's why you set
-- the carry for subtract.  Accumulator + OPERAND' + Carry=1 (the tick
-- means inverted) is the same as Accumulator - OPERAND.  If you don't
-
-So that means the 6502's ALU must have two alternative inputs for the
-second operand: one which presents the true value, and the other which
-presents the inverted value.  (It doesn't do the complement operation
-
-This means a set of eight inverters with tri-state outputs, I suppose.
--- 
-David Empson                                                               

   a set of eight XOR gates would also work. one input to the XOR
gate would be the second operand, the other input would be a subtract
flag (1=subtraction, 0=no subtraction).  this way the second operand
would get inverted when needed.

-- 
Jason Perez                     | "Frodo Lives!"       "Gig 'em!"
j0p7771@tamuts.tamu.edu         | Texas A&M Univ. - it's not just for 
jperez@ee.tamu.edu              |                   farmers anymore!