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| -rw-r--r-- | mu.md | 17 |
1 files changed, 12 insertions, 5 deletions
diff --git a/mu.md b/mu.md index f940a6bd..4abb7610 100644 --- a/mu.md +++ b/mu.md @@ -42,6 +42,13 @@ a `var` statement has two forms: Variables in registers must be initialized. Variables on the stack are implicitly zeroed out. +Variables exist only within the `{}` block they're defined in. Space allocated +to them on the stack is reclaimed after execution leaves the block. Registers +restore whatever variable was using them in the outer block. + +It is perfectly ok to reuse a register for a new variable. Even in a single +block (though you permanently lose the old variable then). + Variables can be in six 32-bit _general-purpose_ registers of the x86 processor. - eax - ebx @@ -50,6 +57,9 @@ Variables can be in six 32-bit _general-purpose_ registers of the x86 processor. - esi ('s' often a mnemonic for 'source') - edi ('d' often a mnemonic for 'destination') +Most functions return results in `eax` by convention. In practice, it ends up +churning through variables pretty quickly. + You can store several types in these registers: - int - boolean @@ -381,7 +391,7 @@ Xor: xor-with var, n ``` -### Shifts +### Bitwise shifts Shifts require variables of non-`addr`, non-`float` types. @@ -415,6 +425,7 @@ You can compute the address of any variable in memory (never in registers): As mentioned up top, `addr` variables can never escape the function where they're computed. You can't store them on the heap, or in compound types. +Think of them as short-lived things. To manage long-lived addresses, _allocate_ them on the heap. @@ -430,10 +441,6 @@ have to first convert them into a short-lived `addr` using _lookup_. var y/eax: (addr T) <- lookup x: (handle T) ``` -The output of `lookup` is always returned in register `eax`. Many other -function calls do the same thing. In practice, this means `eax` ends up being -a temporary location used to store lots of variables in quick succession. - Since handles are large compound types, there's a special helper for comparing them: |