There is no concept of 'delimiting' bytes in SPI. The simple fact that 8 bits have been transmitted constitutes a byte, and the ninth bit would be the first bit of the next byte.

In SPI reception begins when the chip select line is lowered (or raised for some chips). The data is then clocked in one bit at a time into a shift register. As each bit arrives the shift register shuffles all the bits down one.

An SPI chip typically has a fixed shift register size, and is not bound by byte sizes. Some have multiples of 8 bits, which is nice, some have 10 bits, some 17 bits, etc.

If you clock in more than the required number of bits the first bits kind of drop off the end of the shift register and are lost, so if you have a 10 bit shift register, and you can only send multiples of 8 bits (which is common with the PIC chips), then if you send the first byte as 6 bits of 0 followed by 2 bits of data, then a second byte of 8 bits of data, the first 6 bits will be discarded as they drop off the end, and the shift register will only contain the last 10 bits.

Addressing modes are basically taking a few extra bits in the SPI data stream and comparing them to a set of pins tied either high or low on the chip. If they match then the data in the shift register should be acted upon. If they don't then it should be discarded.

A number of SPI chips include a pass-through function where you can chain them together, and as data is clocked into the first chip what is at the end of its shift register, and would normally be discarded, is sent to an output pin. This can then go to the input of the next chip thus passing the data down the line from chip to chip. In this case it is critical to make sure your data is packed into a single stream with no bits that you'd normally discard (can be tricky if the chips don't use multiples of 8 bits).

The number of 'wires' in SPI is misleading at best, as it doesn't really tell you how many real wires are needed.

Typically you have:

• Clock
• Chip Select
• Data in
• Data out

Some chips may not have a data out, and they only accept data into them. Some combine the in and the out together, so you have to split them apart somehow - either in software if you can, or in hardware.

If you have both data in and data out, then SPI can work in full duplex mode (but doesn't always) where as you clock data into the shift register, data is also being clocked out for you to read. This isn't often used, as most systems rely on a command being sent before a response can occur. There is sometimes another line to signal when the data has finished being sent to the SPI device and the response should be sent. More often, though, it happens when a certain number of bits have been received, or a certain combination of bits. It is common to pad the start of a transmission with 0 then signal the device to start receiving with a start bit.

There are many ways of doing it, and no one ever seems to do the same thing as anyone else - or even as themselves sometimes.

SPI defines how the data is transferred, not how the data is formed.

Welcome to the world of consumer electronics and manufacturing in volume! Nobody ever said it made sense!

The difference in price has nothing to do with anything technical. It is purely the economics of the market. The SPI Flash is being sold in relatively low quantities and somewhat high profit margins. The SD card is being sold in huge quantities and a very low profit margin.

While on the surface it might seem that the SD card would be more expensive since it has a smaller capacity and less "middlemen", that obviously isn't the case.

Another complication is that you could buy one make/model of SD card today, and then buy the same make/model in 3 month, and you would not be guaranteed to get the exact same thing. In those 3 months the internal design of the SD card could change. For most consumers this would not matter, but for some embedded users this could kill your application. Also, the SD card maker is not going to tell you of these changes. The same is not true of the SPI Flash, where you will most likely get the same thing for years.

You can get SD cards from manufacturers that will guarantee that they sell the same part for years, but it will be much more expensive.

These things are true of many products, not just SPI Flash and SD Cards. Memory (Flash and RAM) is the most obvious one. Another one is the iPad. In many cases it would be cheaper to buy iPads in bulk than to try and manufacture your own-- even in 100,000 unit quantities. You can't underestimate the purchasing power of a large company building millions of units at a time.

There are other factors that I didn't cover. Differences in part types, packages, purchasing channels, etc. But the problem you raise is more complicated than any one single factor can account for. My market/economic explanation is the biggest factor, but not the only one.