Aesthetically, my favorite architecture in many was is the 14-bit series. The 16-bit PIC18Fxx architecture improves some things, but I find somehow the design less aesthetically pleasing. Which architecture you'll like better probably depends upon your design aesthetic, the extent to which your find yourself wishing things were designed differently, and the extent to which such wishing detracts from your enjoyment working with them.
From a design perspective, there's no particular reason why code addresses and data addresses need to be the same. One thing I like about the 14-bit PICs is that adding a number to an instruction address advances by that many instructions. By contrast, on the PIC18X, each instruction takes two addresses. Consequently, computed jumps using an 8-bit selector are confined to a range of 128 instructions rather than 256. It's a small detail, but having a program counter whose lowest bit is non-functional seems unaesthetic.
Also, the PIC18xx parts add a single-cycle hardware multiply, but unfortunately since it requires one operand to be in W but puts the results in a fixed pair of other registers, it can't be used very effectively for multi-precision operations. If I had my druthers, there would be two types of multiply instructions:
- Simple multiply -- Store W into multiplier register, and store op*W into PRODH:W
- Multply-add --Store PRODH+op*multiplier register into PRODH:W
With such a pattern, a 16x16 operation would be rendered as:
movf OP1L,W
mul OP2L
movwf RESULT0
mula OP2H
movff OP2L,MULTR
mula OP2L
movwf RESULT1
mula OP2H
muvwf RESULT2
movff PRODH,RESULT3
Further, arbitrary-length multiplies could be done with an average cost of a little over two cycles per 8x8 partial product, using the repeated pattern:
mula POSTINC0,c
addwfc POSTINC1,f,c
That pattern would multiply one multi-byte number times an 8-bit value and add the result to another multi-byte number.
As it is, I think the best one can do for an extended multiply is to do the multiply to a destination buffer without doing a built-in add, at a cost of six cycles per 8x8 partial product, and then spend another two-cycles per partial product adding that result to the previous 8xN partial result.
movf multiplier,w
mulwf POSTINC0,c
movf PRODL,w,c
addwfc POSTINC1,w
movff PRODH,INDF1
Four times as long as what could be achieved with a slightly different instruction set. I don't know that I've seen any processor which included a function to compute PRODH+Op1*Op2 but it would be a very simple feature to include in shifter-based multiplies, and it facilitates computing arbitrary product widths with fixed hardware cost. Actually, since the PIC takes four hardware clocks per instruction, the hardware required to allow a 16xN or 32xN multiply would be pretty modest; when computing big products, a 16xN or 32xN multiply with suitable register usage would offer a 2x or 4x speedup.
Best Answer
It's because you're looking at a new chip family. In the 8-bit PICs, the F used to mean flash based (aside from, IIRC, pic16c84 which was the first one) and are therefore more popular than the C series, which had to be erased with UV lights. Some of those didn't even have a window for that, making them one time programmable.
The actual processor families themselves are divided into numbers from PIC 10 to PIC 32 and dsPIC 33; of which 18 and lower are the well known 8-bit PIC family. I think these numbers once indicated the instruction width, but that changed somewhere (pic18f2550 has 16 bit instructions). The PIC24 I haven't looked closely at, but it's some sort of DSP cousin, and PIC32 isn't even based on the PIC processor architecture but MIPS. You'll find the PIC18 and lower are grouped under 8 bit microcontrollers, while the PIC24 is in the 16 bit category along with dsPIC 30 and 33 (yep, that's a higher number than the PIC32 which is 32-bit).
In short, the reason you're finding sample code that doesn't quite apply is because Microchip decided to use its well known PIC brand to market nearly all their microcontrollers. The effect you suffer from is known as brand dilution.
For a comparison, it's like the 10..18 families are like x86 processors from the 386 to the Pentium II, each time adding some instructions but largely similar to program, but the PIC24 is the sudden shift to a dissimilar IA-64 instruction set (Itanium) and the PIC32 is a third party product (like StrongARM). All those were made by Intel, but they avoided this level of confusion by not (at the time) branding the Itanium as a Pentium or Xeon device. AMD, on the other hand, is about to add some confusion with ARM architecture Opteron brand chips.