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215 lines
3.7 KiB
HTML
215 lines
3.7 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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><TITLE
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>A quick digression on how subroutines work</TITLE
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CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
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TITLE="Programming with Ophis"
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TITLE="Dispatch-on-type and Data-Directed Assembler"
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>Functionals</TD
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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="AEN836"
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>A quick digression on how subroutines work</A
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></H1
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><P
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> Ordinarily, subroutines are called with <TT
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CLASS="LITERAL"
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>JSR</TT
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> and
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finished with <TT
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CLASS="LITERAL"
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>RTS</TT
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>. The <TT
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CLASS="LITERAL"
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>JSR</TT
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>
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instruction takes its own address, adds 2 to it, and pushes this
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16-bit value on the stack, high byte first, then low byte (so that
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the low byte will be popped off first).
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</P
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><P
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> But wait, you may object. All <TT
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CLASS="LITERAL"
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>JSR</TT
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> instructions
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are three bytes long. This <SPAN
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CLASS="QUOTE"
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>"return address"</SPAN
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> is in
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the middle of the instruction. And you would be quite right;
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the <TT
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CLASS="LITERAL"
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>RTS</TT
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> instruction pops off the 16-bit
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address, adds one to it, and <I
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CLASS="EMPHASIS"
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>then</I
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> sets the
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program counter to that value.
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</P
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><P
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> So it <I
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CLASS="EMPHASIS"
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>is</I
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> possible to set up
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a <SPAN
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CLASS="QUOTE"
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>"<TT
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CLASS="LITERAL"
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>JSR</TT
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> indirect"</SPAN
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> kind of operation
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by adding two to the indirect jump's address and then pushing that
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value onto the stack before making the jump; however, you wouldn't
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want to do this. It takes six bytes and trashes your accumulator,
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and you can get the same functionality with half the space and
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with no register corruption by simply defining the indirect jump
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to be a one-instruction routine and <TT
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CLASS="LITERAL"
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>JSR</TT
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>-ing to
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it directly. As an added bonus, that way if you have multiple
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indirect jumps through the same pointer, you don't need to
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duplicate the jump instruction.
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</P
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><P
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> Does this mean that abusing <TT
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CLASS="LITERAL"
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>JSR</TT
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>
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and <TT
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CLASS="LITERAL"
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>RTS</TT
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> is a dead-end, though? Not at all...
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</P
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></DIV
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><<< Previous</A
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WIDTH="33%"
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>Functionals</TD
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><TD
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WIDTH="34%"
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>Up</A
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>Dispatch-on-type and Data-Directed Assembler</TD
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> |