Pop65

Pop65 is a simple, bare-bones 6502 assembler. I wrote it to use myself, since most 6502 assemblers on modern platforms add many features; Pop65 is more like the old 8-bit ones. I also wanted to be able to write a LSP server for it — this has not yet been developed, but should be doable with minimum modifications, including the easy ability to check the value of a symbol and go to its definition.

Pop65 outputs bytes as they're encountered, ignoring the .org commands except to set the current internal address. For example, if you wanted to develop an NES game, you would .org 0 and output the header, then .org $8000 and output the first bank, .org $8000 and output the second bank, etc. You can fill the empty space at the end of a bank by going .ds *-$C000, for example.

Line Format

Lines are formatted as:

[{label}[:]] [{operation}] [;{comment}]

Whitespace is ignored outside of strings.

Expressions

All expressions are in unsigned 16-bits, with overflow ignored ($FFFF+1=0).

The operators are, in order of precedence (highest to lowest):

1. </>: unary. Get the low/high byte of the following expression.
2. </>/<=/>=/=/<>/><: binary, relational. Takes its two arguments and compares less than/greater/less than or equal/greater or equal, equal to, or two forms of not equal.
3. +/-: binary. Add or subtract.
4. *///%: binary. Multiply, divide, modulo.
5. -: unary. Negate (flip all bits and add 1).

You can change grouping with parenthesis.

You can specify the base of a numeric constant like this:

• $: hexadecimal (0-9,A-F), case is ignored.
• %: binary (0-1).
• @: ocatl (0-7).

A string can be enclosed in either '...' or "...", as long as the right quote matches the left quote. At present, no string escapes are allowed. A one character string may be employed anywhere a constant integer might; for instance, '3' evaluates to $33, and "9" evaluates to $39.

The * symbol evaluates to the present Program Counter.

Pseudo-Ops

All pseudo-ops start with a . character. Case is ignored.

• .if {expr}: If the expression evaluates to zero, everything up until the matching .endif is skipped, and not assembled. The expression must be evaluated in the first pass (no forward references).
• .else: the sense of the closest matching active .if is flipped; .if 0 foo .else bar .endif would assemble bar.
• .endif: ends the closest matching active .if.
• .assert {expr}: if the expression evaluates to zero, the assembler will issue an assertion error. The expression is only evaluated in the second pass, so forward references are allowed.
• .dbg {string}/.dbg: the current debug format string is set (see below). Without a string (in the second form), it disabled debug output until set again.
• .ds {expr1} [, {expr2}]: places expr1 bytes in the output. If expr2 is provided, it is evaluated and its value is used; otherwise, 0 is used instead. For example, .ds 2 outputs 0,0, while .ds 3,4 outputs 4,4,4.
• .bin {string}/.incbin {string}: the file with the string name is loaded, and its raw bytes placed into the output.
• .inc {string}/.lib {string}/.fil {string}: the file with the string name is treated as a new assembly file and included here.
• {label} = {expr}/{label} .equ {expr}: assign the label a specific value. The expression must be evaluated in the first pass (no forward references). Labels created in this way are not sent to the debug file, but are sent to the symbol table file.
• .org {expr}: set the Program Counter to the value; the expression must be evaluated in the first pass (no forward references).
• .byte {expr} , {expr} ...: evaluate each expression and place it as a single byte in the output file.
• .word {expr} , {expr} ...: evaluate each expression and place it as a 16-bit little endian word in the output file.
• .off: disable output of any bytes; useful for generating RAM labels via .ds pseudo-ops.
• .on: enable output of any bytes; see .off.

Filename strings can use Windows or Unix style path seperators (/ or \\) interchangably.

Debug File

Pop65 can be set to output to a "debug file." At any time, the .dbg pseudo-op can be used to set a debug format string, or to disable debug output (debugging starts disabled).

The string is output directly, with special escapes contained in braces. Here are the escapes:

• {L}: outputs the label name.
• {C}: outputs the surrounding comments for the label, with newlines replaced by spaces.
• {V}: output the 16-bit value of the label in hexadecimal, with leading 0's stripped out.
• {V(signed hex number)}: (for instance, {V3FF}). This adds a signed, 32-bit hexadecimal number to the unsigned, 16-bit value of the label, and outputs that in hexadecimal here.

For instance, code:

.org $8000
.dbg "P:{V-8000}:{L}:{C}"
foo .word bar   ; description of foo

; description of...
; bar!
bar .word foo

Would output to the debug file:

P:0:foo:description of foo
P:2:bar:description of... bar!

Which is the proper debug format for the Mesen emulator.