rx.muri: attempt to clarify some things with the need to use nop after instructions changing ip; fix a typo; remove magic numbers from the primitives
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@ -23,7 +23,7 @@ This source is written in Muri, an assembler for Nga.
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Before going on, I should explain a bit about Nga and Muri.
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Nga provides a MISC inspired virtual machine for a dual stack
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architecture. There are 27 instructions, with up to four packed
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architecture. There are 30 instructions, with up to four packed
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into each memory location (*cell*). The instructions are:
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0 nop 5 push 10 ret 15 fetch 20 div 25 zret
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@ -42,7 +42,6 @@ RETURN, and ZRET. Additionally, if the instruction bundle
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contains a LIT, a value must be in the following cell. (One for
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each LIT in the bundle)
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The reason for this relates to how Nga processes the opcodes.
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To illustrate, assume a stack with a couple of values:
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@ -51,28 +50,40 @@ To illustrate, assume a stack with a couple of values:
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And a function that consumes two values before returning a
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some new ones:
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:function * #3 ;
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: function
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i mulire..
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d 3
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If we were to use an instruction bundle like:
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(In the `i` line, the instructions are mul, lit, return, and
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nop).
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lit call add nop
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function
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If we were to use this along with an instruction bundle like:
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Nga will:
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: test
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i licaadre
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r function
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(1) push a pointer to the function to the stack
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(In the `i` line, the instructions are lit, call, add, and
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return).
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On running the code in `test` Nga will:
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(1) push a pointer to `function` to the stack
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(2) setup a call to the function
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(3) add the top values on the stack (#1 #2),
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leaving a single value (#3)
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(4) do nothing
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(4) setup a return to the caller
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At this point the bundle is done, so control goes to the
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called function. But we now have only one value on the stack,
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so the stack underflows and Nga will crash.
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It's not forbidden, and this can be useful to improve code
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density, but exercise caution and be sure to keep track of
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this behavior to avoid hard to identify bugs.
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It's not forbidden by my VM implementation, but John's Impexus
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implementation differs here in that it does not process the
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instructions after jump, call, ccall, ret, or zret. I really
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recommend making sure your bundles don't contain anything after
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these instructions to keep things portable and predictable in
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behavior.
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Muri uses the first two characters of each instruction name
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when composing the bundles, with NOP being named as two dots.
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@ -88,7 +99,7 @@ Is a bundle named:
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And with two `li` instructions, must be followed by two values.
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Muri uses a directive in the first line to tell it what to
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expect. Directives are:
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expect. Valid directives are:
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i instruction bundle
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d decimal value
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@ -156,107 +167,107 @@ Many will also be exposed in the initial dictionary.
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~~~
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: _nop
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d 0
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i ........
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i re......
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: _lit
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d 1
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i li......
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i re......
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: _dup
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d 2
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i du......
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i re......
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: _drop
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d 3
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i dr......
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i re......
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: _swap
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d 4
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i sw......
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i re......
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: _push
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d 5
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i pu......
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i re......
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: _pop
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d 6
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i po......
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i re......
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: _jump
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d 7
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i ju......
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i re......
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: _call
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d 8
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i ca......
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i re......
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: _ccall
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d 9
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i cc......
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i re......
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: _ret
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d 10
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i re......
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i re......
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: _eq
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d 11
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i eq......
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i re......
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: _neq
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d 12
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i ne......
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i re......
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: _lt
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d 13
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i lt......
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i re......
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: _gt
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d 14
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i gt......
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i re......
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: _fetch
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d 15
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i fe......
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i re......
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: _store
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d 16
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i st......
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i re......
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: _add
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d 17
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i ad......
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i re......
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: _sub
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d 18
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i su......
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i re......
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: _mul
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d 19
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i mu......
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i re......
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: _divmod
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d 20
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i di......
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i re......
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: _and
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d 21
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i an......
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i re......
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: _or
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d 22
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i or......
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i re......
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: _xor
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d 23
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i xo......
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i re......
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: _shift
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d 24
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i sh......
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i re......
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: _zret
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d 25
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i zr......
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i re......
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~~~
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