allisonlastname wrote:
What is it with ICs having all 8 data pins (mostly) contiguous, and then the lower order address pins contiguous, and then the high order address pins all over the place? The 6502 doesn't do it but the 39SF010 and the 62256 both do.
It makes some sense when you look at the history of the pinouts. Start with a 1K x 8 chip like the 2708: this comes in a 24 pin package, with the data pins nicely in order around the bottom, and the address pins nicely in order around the top:
Code:
A7 VCC
A6 A8
A5 A9
A4 VEE
A3 /CE
A2 VDD
A1 PGM
A0 D7
D0 D6
D1 D5
D2 D4
GND D3
Then you move on to the 2K x 8 2716. This needs one more address pin, but no longer needs the -5V supply. So A10 takes the place of VDD. (It also only needs high voltage during programming, so VEE becomes VPP)
Now it's the 4K x 8 2732. We need another address pin, so there's a bit more shuffling among the other pins to make room for A11 where VPP used to be. The pinout now looks like this:
Code:
A7 VCC
A6 A8
A5 A9
A4 A11
A3 /OE / VPP
A2 A10
A1 /CE
A0 D7
D0 D6
D1 D5
D2 D4
GND D3
Density increases, we get more capacity, but we've run out of pins. So we add some more at the top and the 2764 pinout looks like this:
Code:
VPP VCC
A12 /PGM
A7 NC
A6 A8
A5 A9
A4 A11
A3 /OE
A2 A10
A1 /CE
A0 D7
D0 D6
D1 D5
D2 D4
GND D3
There's lots more room, so the doubled up /OE and VPP have moved to different pins, /PGM is back, and there's even an unused pin. Guess what happens with the 27128? That's right, that unused pin becomes A13.
Next comes the 27256, and we've run out of pins again. PGM goes, and A14 takes its place. For the 27512, VPP moves back to sharing with /OE, and A15 takes over pin 1.
It's a pattern that we see all too often in computing. The final design makes no sense in itself, but only as the end-product of a series of short-term decisions. With each chip, the new functions are squeezed in wherever they will fit, with no concern for what's going to happen on the next one.