new wiring through attic framing

I am currently replacing a single knob-and-tube circuit for our second floor with four new circuits. I ran the new wiring up through a chase into the attic, where all four lines end in their own junction box (1 box per room). The joists in the attic are odd shapes though; every second joist is a full 1x6, each alternating joist is a full 2x4. When I need to run wiring perpendicular to the joists can I fasten the wire to the top of the joist? or do I need to drill a hole in each one? And, if I need to drill holes, can I pick a single height (say, two inches from the bottom of each joist) or do I need to center each hole on a given joist?

Your answers will be much appreciated. Thanks

 

see 330.24 in the nec.
320.23 In Accessible Attics.
Type AC(BX) and NM(romex) cables in accessible attics or roof spaces shall be installed as specified in 320.23(A) and (B).
(A) Where Run Across the Top of Floor Joists. Where run across the top of floor joists, or within 2.1 m (7 ft) of floor or floor joists across the face of rafters or studding, in attics and roof spaces that are accessible, the cable shall be protected by substantial guard strips that are at least as high as the cable. Where this space is not accessible by permanent stairs or ladders, protection shall only be required within 1.8 m (6 ft) of the nearest edge of the scuttle hole or attic entrance.
In accessible attics, Type AC cable installed across the top of floor joists or within 7 ft of the floor or floor joists across the face of rafters or studs must be protected by guard strips. Where the attic is not accessible by a permanent ladder or stairs, guard strips are required only within 6 ft of the scuttle hole or opening.
(B) Cable Installed Parallel to Framing Members. Where the cable is installed parallel to the sides of rafters, studs, or floor joists, neither guard strips nor running boards shall be required, and the installation shall also comply with 300.4(D). When you install parallel with joist, keep them at least an 1-1/4" from the surface. when you install thru the joist, also keep them 1-1/4" from the surface.

 

To summarize the codes that whitey2 cited, just run the cables on top of the joists, but install a 2x4 along side of it (to keep someone from stepping on the cable itself).

 

Thanks for the info. It will make my project go alot smoother.