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OS_CPU.H, OS_ENTER_CRITICAL() and OS_EXIT_CRITICAL()
OS_CPU.H, Stack Growth
(11) The stack on an 80x86 processor grows from high to low memory, which means that OS_STK_GROWTH must be set to 1.
OS_CPU.H, OS_TASK_SW()
(13) To switch context, OS_TASK_SW() needs to simulate an interrupt. The 80x86 provides 256 software interrupts to accomplish this. The interrupt service routine (ISR) (also called the exception handler) must vector to the assembly language function OSCtxSw() (see OS_CPU_A.ASM). We thus need to ensure that the pointer at vector 0x80 points to OSCtxSw().
(12) I tested the code on a PC and I decided to use interrupt number 128 (0x80) because I found it to be available. Actually, the original PC used interrupts 0x80 through 0xF0 for the BASIC interpreter. Few if any PCs come with a BASIC interpreter built in anymore so it should be safe to use these vectors. Optionally, you can also use vectors 0x4B to 0x5B, 0x5D to 0x66, or 0x68 to 0x6F. If you use this port on an embedded processor such as the 80186, you will most likely not be as restricted in your choice of vectors.
OS_CPU.H, Tick Rate
The tick rate for an RTOS should generally be set between 10 and 100Hz. It is always preferable (but not necessary) to set the tick rate to a round number. Unfortunately, on the PC, the default tick rate is 18.20648Hz, which is not what I would call a nice round number. For this port, I decided to change the tick rate of the PC from the standard 18.20648Hz to 200Hz (i.e., 5ms between ticks). There are three reasons to do this:
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The spreadsheet is actually provided on in the companion CD (\SOFTWARE\downloadable package (uCOS-II\Ix86L\BC45\DOC\80x86L-ROM-RAM-RAM-Calc.XLS). You need Microsoft Excel for Office 2000 (or higher) to use this file. The spreadsheet allows you to do “what-if” scenarios based on the options you select. You can change the configuration values (in RED) and see how they affects µC/OS-II’s ROM and RAM usage on the 80x86. For the ???_EN values, you MUST use either 0 or 1.
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