# Constant expressions (ivss)

**Constant expressions** assign computed absolute values to symbols through `.equ`. They can combine literals and known symbols using arithmetic and bitwise operators:

```asm
.section my_data
begin:
    .word 11, 22, 33
finish:
.equ byte_count, finish - begin
.equ word_count, byte_count / 4
.equ mask, (1 << 3) - 1
```

The labels are twelve bytes apart, so `byte_count` becomes twelve and `word_count` becomes three. `mask` becomes seven, setting its lowest three bits. These definitions do not append bytes to the section.

ivss stores `.equ` requests while reading source, then evaluates them after discovering labels. Requests are processed in their recorded order, so this example defines `byte_count` before using it in `word_count`.

The expression parser handles precedence and parentheses using operator and value stacks. A completed result becomes an absolute symbol. Subtracting offsets from the same section gives a size independent of that section's eventual base address; a label's offset alone is not its final linked address. Use ordinary symbol references when relocation must supply that address.

Source: [asembler.c](https://github.com/yanevskiv/ivss/blob/main/src/asembler.c).