I don't understand how an app (e.g. Photoshop) could take advantage of an SMP. I mean, doesn't the OS "shield" the hardware from the apps? Applicantions are not supposed to know about whether it is a uniprocessor, an SMP, or a dual-core, right? And what if the app knows it's SMP or dual-core, but the OS doesn't support SMP or dual core? Is it possible? Thx.

The OS doesn't do any more shielding from apps these days than buffer overflow protection with the NX/EDB bits on Intel/AMD processors. Applications, the OS, and the hardware is designed to let the application know if SMP exists explicitly for the purpose of taking advantage of it. Applications are most certainly supposed to know. Additionally, if the OS doesn't support SMP, the application has no idea -- it's all fed directly from the OS.

sorry that i have to make a comment about 64 bit processors here, because there is no 'comment' feature in the "64-bit simplified" article.
What on earth is a 64-bit processor? Does it mean 64-bit data bus? 64-bit control bus? 64-bit registers? 64-bit everything?

In computers, a 64-bit processor is one in which data is processed and/or stored in 64-bit chunks. The term often refers to the size of the CPU's registers that holds memory adddresses and other data, as well as the arithmetic logic unit that operates on those registers to compute. Busses have been varying widths for ages, with the Athlon XP being a 32 bit CPU with a 64 bit bus (Twin 32 bit pipelines) -- the Athlon 64 has a 64 bit memory-to-CPU bus, and a hypertransport bus which is CPU-to-chipset and it's 16bit bi-directional. A computer architecture described as "64-bit" generally has integer registers that are 64 bits wide and thus directly supports dealing both internally and externally with 64-bit "chunks" of data.

Bus widths don't count in the equation.