rtd DM5816, User Manual

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DM5816/DM6816 RTD Embedded Technologies, Inc.
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inportb(BaseAddress + 16); /* Clear jumper selectable interrupt */
outportb(0x20, 0x20); /* Send EOI command to 8259 */
}
In Pascal:
Procedure ISR; Interrupt;
begin
{ Your code goes here.
Do not
use any DOS functions! }
c := Port[BaseA 16]; { Clear jumper selectable interrupt }
Port[$20] := $20; { Send EOI command to 8259 }
end;
Saving the Startup Interrupt Mask Register (IMR) and Interrupt Vector
The next step after writing the ISR is to save the startup state of the interrupt mask register and
the interrupt vector that you will be using. The IMR is located at I/O port 21H. The interrupt
vector you will be using is located in the interrupt vector table which is simply an array of 256-bit
(4-byte) pointers and is located in the first 1024 bytes of memory (Segment = 0, Offset = 0). You
can read this value directly, but it is a better practice to use DOS function 35H (get interrupt
vector). Most C and Pascal compilers provide a library routine for reading the value of a vector.
The vectors for the hardware interrupts are vectors 8 through 15, where IRQ0 uses vector 8,
IRQ1 uses vector 9, and so on. Thus, if the DM5816 will be using IRQ3, you should save the
value of interrupt vector 11.
Before you install your ISR, temporarily mask out the IRQ you will be using. This prevents the
IRQ from requesting an interrupt while you are installing and initializing your ISR. To mask the
IRQ, read in the current IMR at I/O port 21H and
set
the bit that corresponds to your IRQ
(remember, setting a bit disables interrupts on that IRQ while clearing a bit enables them). The
IMR is arranged so that bit 0 is for IRQ0, bit 1 is for IRQ1, and so on. See the paragraph
entitled Interrupt Mask Register (IMR) earlier in this chapter for help in determining your IRQ’s
bit. After setting the bit, write the new value to I/O port 21H.
With the startup IMR saved and the interrupts on your IRQ temporarily disabled, you can assign
the interrupt vector to point to your ISR. Again, you can overwrite the appropriate entry in the
vector table with a direct memory write, but this is a bad practice. Instead, use either DOS
function 25H (set interrupt vector) or, if your compiler provides it, the library routine for setting an
interrupt vector. Remember that vector 8 is for IRQ0, vector 9 is for IRQ1, and so on.
If you need to program the source of your interrupts, do that next. For example, if you are using
the program- mable interval timer to generate interrupts, you must program it to run in the
proper mode and at the proper rate.
Finally, clear the bit in the IMR for the IRQ you are using. This enables interrupts on the IRQ.
Restoring the Startup IMR and Interrupt Vector