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GETTING STARTED WITH EMBEDDED PCs

by Bob Perrin

Start ý Selecting an Embedded PC ý Hello World ý Controlling Resources ý Thank You, Intel ý Sources and PDF

THANK YOU, INTEL

The code in Listing 1 uses the MK_FP macro to initialize the pointers. MK_FP accepts two parameters: a SEGMENT and an OFFSET. Some people may not be clear on exactly what these parameters are. Allow me to explain.

For some reason, back in the 1980s, Intel decided that having a memory space accessed by a single 20-bit address register was not the most optimal method. Nevermind the fact that Motorola's MC68000 family had a 24-bit address space accessed by using 32-bit internal address registers (the most significant 8 bits were not put out on the external 24-bit address bus). Instead, Intel decided to use two 16-bit values (a SEGMENT and an OFFSET) to compute a 20-bit physical address.

The computation is simple. Left shift the SEGMENT a nibble and add it to the OFFSET. The least significant 20 bits of the sum comprise the physical address. Although that all seems pretty easy, consider that there is no longer only one unique representation of a physical address. As long as the bits in the SEGMENT and OFFSET values add to the desired 20-bit sum, that combination of SEGMENT and OFFSET registers is a legitimate representation of the physical address.

Figure 1 shows how the SEGMENT and OFFSET are aligned to compute a physical address.

My understanding of Intel's reasoning for this scheme was that the SEGMENT/OFFSET method would make multitasking easier. Segments could be assigned to separate tasks and, when required, segments could overlap. Unfortunately, 15 years later we still have to live with the legacy of this kooky idea.

APPLICATION TERMINATED

Embedded PCs can seem a bit intimidating at first look, but like most things, once you have one on your desk to play with, the mist lifts and the picture becomes clear. Starting with a simple board and taking things one step at a time makes the learning easier. The bottom line seems to be, if you can write a bit of code in C and can read a datasheet, you can use an embedded PC.

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