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172 lines
3.1 KiB
HTML
172 lines
3.1 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<HTML
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><HEAD
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><TITLE
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>The stack</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
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REL="HOME"
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TITLE="Programming with Ophis"
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HREF="book1.html"><LINK
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REL="UP"
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TITLE="Structured Programming"
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HREF="c543.html"><LINK
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REL="PREVIOUS"
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TITLE="Structured Programming"
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HREF="c543.html"><LINK
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REL="NEXT"
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TITLE="Procedures and register saving"
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HREF="x603.html"></HEAD
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><BODY
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CLASS="SECTION"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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><TR
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><TH
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COLSPAN="3"
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>Programming with Ophis</TH
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><TR
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WIDTH="10%"
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Structured Programming</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECTION"
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><H1
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CLASS="SECTION"
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><A
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NAME="AEN597"
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>The stack</A
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></H1
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><P
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> The 6502 has an onboard stack in page 1. You can modify the stack
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pointer by storing values in X register and
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using <TT
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CLASS="LITERAL"
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>txs</TT
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>; an <SPAN
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CLASS="QUOTE"
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>"empty"</SPAN
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> stack is
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value $FF. Going into a procedure pushes the address of the next
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instruction onto the stack, and RTS pops that value off and jumps
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there. (Well, not precisely. JSR actually pushes a value that's
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one instruction short, and RTS loads the value, increases it by
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one, and THEN jumps there. But that's only an issue if you're
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using RTS to implement jump tables.) On an interrupt, the next
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instruction's address is pushed on the stack, then the process
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flags, and it jumps to the handler. The return from interrupt
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restores the flags and the PC, just as if nothing had
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happened.
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</P
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><P
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> The stack only has 256 possible entries; since addresses take two
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bytes to store, that means that if you call something that calls
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something that calls something that (etc., etc., 129 times), your
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computation will fail. This can happen faster if you save
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registers or memory values on the stack (see below).
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</P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><A
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HREF="c543.html"
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><<< Previous</A
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>Home</A
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HREF="x603.html"
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>Next >>></A
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></TD
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></TR
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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>Structured Programming</TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="c543.html"
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>Up</A
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WIDTH="33%"
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VALIGN="top"
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>Procedures and register saving</TD
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></TR
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></TABLE
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></DIV
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></BODY
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></HTML
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> |