39 lines
787 B
Markdown
39 lines
787 B
Markdown
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+++
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title = "`type`"
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weight = 1
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+++
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Defines a type synonym. These are used purely for readability, *i.e.,* a
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type synonym can always be "expanded out" to its definition at any time.
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```
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topDefn ::= type typeId {tyVarId }= type
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```
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Examples:
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```hs
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type Byte = Bit 8
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type Word = Bit 16
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type LongWord = Bit 32
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```
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These provide commonly used names for certain bit lengths. In a
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specification of a processor:
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```hs
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data RegName = R0 | R1 | ... | R31
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type Rdest = RegName
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type Rsrc = RegName
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data ArithInstr = Add Rdest Rsrc Rsrc
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| Sub Rdest Rsrc Rsrc
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```
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the last two lines suggest the roles of the registers in the
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instructions, and is more readable than:
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```hs
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data ArithInstr = Add RegName RegName RegName
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| Sub RegName RegName RegName
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```
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