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172 lines
6.0 KiB
Plaintext
172 lines
6.0 KiB
Plaintext
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ACME
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...the ACME Crossassembler for Multiple Environments
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--- addressing modes ---
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If an instruction can be used with different addressing modes, ACME
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has to decide which one to use. Several instructions of the 6502 CPU
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can be used with either "absolute" addressing or "zeropage-absolute"
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addressing. The former one means there's a 16-bit argument, the latter
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one means there's an 8-bit argument.
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And the 65816 CPU even has some instructions with 24-bit addressing...
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So how does ACME know which addressing mode to use?
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The simple approach is to always use the smallest possible argument,
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of course: If the argument fits in a byte, use zeropage addressing. If
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it doesn't, use absolute addressing. If it needs more than two bytes
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and the 65816 CPU is chosen, use 24-bit addressing.
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In most cases this works - with two exceptions. The remainder of this
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text may sound somewhat confusing now, so if you don't have any
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problems with addressing modes, then don't bother trying to understand
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everything this texts says. :)
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The two exceptions are:
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*** 1) Symbols are defined too late
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If ACME cannot figure out the argument value in the first pass, it
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assumes that the instruction uses 16-bit addressing.
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If it later finds out that the argument only needs 8 bits, ACME gives
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a warning ("using oversized addressing mode") and continues. However,
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if it finds out that the argument needs 24 bits, it gives an error.
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These problems can be solved by defining the symbols *before* using
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them, so that the value can be figured out in the first pass. If this
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is not possible, you can use the postfix method, effectively exactly
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defining what addressing mode to use. The postfix method is described
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in a separate section below.
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*** 2) You *want* to use an oversized addressing mode
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On the 65816 CPU, "zeropage addressing" is called "direct page
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addressing". The difference is that the position of the "direct page"
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can be changed. Then, "lda $fa" does not necessarily access the same
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memory location as "lda $00fa" anymore. The same goes for 16- and 24-
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bit addressing: "lda $fabc" does not necessarily access the same
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memory location as "lda $00fabc", because the default bank can be set
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to something other than zero.
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But even on the plain 6502 CPU you might want to force ACME to use an
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oversized addressing mode, for example because of timing issues.
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Again there are two ways to solve the problem: You can define the
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target location using leading zeros. ACME will then use an addressing
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mode that is big enough even if the leading zeros would have been
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other digits:
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symbol1 = $fb
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symbol2 = $00fd
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symbol3 = $0000ff
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lda $fa
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sta $00fc
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lda $0000fe
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sta symbol1
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lda symbol2
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sta symbol3
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will be assembled to
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a5 fa ; lda $fa
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8d fc 00 ; sta $00fc
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af fe 00 00 ; lda $0000fe
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85 fb ; sta $fb
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ad fd 00 ; lda $00fd
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8f ff 00 00 ; sta $0000ff
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This feature can be disabled using the "--ignore-zeroes" CLI switch.
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The other possibility is to use the postfix method (described in the
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next section).
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*** The postfix method
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Warning: This may sound very complicated at first, but I think that
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once you get used to it you'll agree it's useful. If you don't want to
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use this, stick to the "leading zeros" method and don't bother about
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postfixes.
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Still with me? Okay:
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You can force ACME to use a specific addressing mode by adding "+1",
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"+2" or "+3" to the assembler mnemonic. Each one of these postfixes
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sets the relevant "Force Bit" in ACME's result. If Force Bit 3 is set,
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ACME will use 24-bit addressing. Force Bit 2 means 16-bit addressing
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and Force Bit 1 means 8-bit addressing. Higher Force Bits have higher
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priorities.
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Here's an (overly complicated) example:
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symbol1 = $fb
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symbol2 = $fd
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symbol3+3 = $ff ; set Force Bit 3 and store in symbol's flags
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ldx $fa
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ldy+2 $fc ; set Force Bit 2 (16-bit addressing)
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lda+3 $fe ; set Force Bit 3 (24-bit addressing)
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stx symbol1
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sty+2 symbol2 ; set Force Bit 2 (16-bit addressing)
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sta symbol3 ; no need to set Force Bit 3 as it is
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; already set in "symbol3".
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will be assembled to
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a6 fa ; ldx $fa
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ac fc 00 ; ldy $00fc
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af fe 00 00 ; lda $0000fe
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86 fb ; stx $fb
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8c fd 00 ; sty $00fd
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8f ff 00 00 ; sta $0000ff
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Postfixes added directly to the mnemonic have higher priority than
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those given to the argument. As you can see, you can add the postfix
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to the symbol definition as well (equivalent to leading zeros).
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Applying the byte extraction operators ("<" gives the low byte, ">"
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gives the high byte and "^" gives the bank byte of a value) to any
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value will clear the argument's Force Bits 2 and 3 and set Force
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Bit 1 instead. So "lda <symbol" will use 8-bit addressing, regardless
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of the symbol's Force Bits. Of course, you can change this by
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postfixing the instruction again... :)
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*** The algorithm to find the right addressing mode
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You don't need to read this paragraph just to use ACME, I only
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included it for completeness' sake. This is a description of ACME's
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strategy for finding the addressing mode to use:
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First, ACME checks whether the instruction has any postfix. If it has,
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ACME acts upon it. So postfixes have the highest priority.
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Otherwise, ACME checks whether the argument has any Force Bits set
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(because of leading zeros or byte extraction operators, for example)
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or references symbols that have. If any Force Bit is set, ACME acts
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upon it. This is the next priority level: Leading zeros or postfixes
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added to the symbol definition.
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Otherwise, ACME checks whether the argument could be figured out in
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the first pass. If it couldn't, ACME tries to use a default addressing
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mode (normally this will be 16-bit addressing).
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In case the value could be figured out even in the first pass, then
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everything's cool and froody: ACME just looks at the argument's value
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and uses the smallest addressing mode that matches.
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I admit that this algorithm sounds complicated, but it hasn't failed
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yet. :) And in everyday usage, it does exactly what one expects.
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If you want to take a closer look at the algorithm, examine the
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"calc_arg_size" function in the "src/mnemo.c" file.
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