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<a name="1142987"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">D.5 ST20TP3 and ST20TP4</font>
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<a name="1142988"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">A problem is sometimes encountered with DMA engines being locked out. Sometimes a memory lock signal is left asserted after taking an interrupt, which can lock out DMA from peripherals. The lock can be freed by adding a device read to external memory at the start of all trap handlers.</font>
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<a name="1142989"> </a><font color="#003366" face="Verdana, Arial, Helvetica, sans-serif">Work-around</font>
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<a name="1142990"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The OS20 interrupt handler code contains a work-around, which needs to read external memory. </font>
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<a name="1142991"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">OS20 users need to place a memory section named <font size=2 face=Courier><strong>os20_lock_fix</strong></font> in external memory. Since the section is not written to, it may be placed in ROM.</font>
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<a name="1144328"> </a><font size=2 face="Verdana, Arial, Helvetica, sans-serif">The interrupt handler uses device instructions to read from this memory location, so it doesn't matter whether the location is cacheable or not.</font>
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