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<H3><A NAME="Heading8"></A><FONT COLOR="#000077">Notes on Set/Reset</FONT></H3>
<P>The set/reset circuitry is not active in write modes 1 or 2. The Enable Set/Reset register is inactive in write mode 3, but the Set/Reset register provides the primary drawing color in write mode 3, as discussed in the next chapter.
</P>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/25-02i.jpg"><TD WIDTH="95%"><SMALL><I>Be aware that because set/reset directly replaces CPU data, it does not necessarily have to force an entire display memory byte to 0 or 0FFH, even when set/reset is replacing CPU data for all planes. For example, if the Bit Mask register is set to 80H, the set/reset circuitry can only modify bit 7 of the destination byte in each plane, since the other seven bits will come from the latches for each plane. Similarly, the set/reset value for each plane can be modified by that plane&#146;s ALU. Once again, this illustrates that set/reset merely replaces the CPU data for selected planes; the set/reset value is then processed in exactly the same way that CPU data normally is.</I></SMALL>
<TABLE WIDTH="100%"><TD WIDTH="5%" VALIGN="TOP"><IMG SRC="images/i.jpg"><TD WIDTH="95%"><SMALL><I>Be aware that because set/reset directly replaces CPU data, it does not necessarily have to force an entire display memory byte to 0 or 0FFH, even when set/reset is replacing CPU data for all planes. For example, if the Bit Mask register is set to 80H, the set/reset circuitry can only modify bit 7 of the destination byte in each plane, since the other seven bits will come from the latches for each plane. Similarly, the set/reset value for each plane can be modified by that plane&#146;s ALU. Once again, this illustrates that set/reset merely replaces the CPU data for selected planes; the set/reset value is then processed in exactly the same way that CPU data normally is.</I></SMALL>
</TABLE>
<H3><A NAME="Heading9"></A><FONT COLOR="#000077">A Brief Note on Word OUTs</FONT></H3>
<P>In the early days of the EGA and VGA, there was considerable debate about whether it was safe to do word <B>OUT</B>s (<B>OUT DX,AX</B>) to set Index/Data register pairs in a single instruction. Long ago, there were a few computers with buses that weren&#146;t quite PC-compatatible, in that the two bytes in each word <B>OUT</B> went to the VGA in the wrong order: Data register first, then Index register, with predictably disastrous results. Consequently, I generally wrote my code in those days to use two 8-bit <B>OUT</B>s to set indexed registers. Later on, I made it a habit to use macros that could do either one 16-bit <B>OUT</B> or two 8-bit <B>OUT</B>s, depending on how I chose to assemble the code, and in fact you&#146;ll find both ways of dealing with <B>OUT</B>s sprinkled through the code in this part of the book. Using macros for word OUTs is still not a bad idea in that it does no harm, but in my opinion it&#146;s no longer necessary. Word <B>OUT</B>s are standard now, and it&#146;s been a long time since I&#146;ve heard of them causing any problems.</P><P><BR></P>