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Schemta - The Scheme Multi Target Assembler

About

Schemta is a cross-platform, multi-target assembler for classic 8-bit and 16-bit architectures. It uses embedded Scheme code as a powerful replacement for traditional assembler macros. Schemta currently supports the following architectures:

  • Fairchild F8 (F3850)
    • register/addressing aliases: pc0 for p/p0, (is), (is+), (is-) for s, i, d
  • MOS Technology 6502/6507/6510
    • supports all illegal opcodes
    • assumes absolute addressing for labels, force zero page with <LABEL>
  • Motorola 6809
    • supports undocumented opcodes hcf, ncc (neg + carry bit), page, and reset, as well as immediate addressing for stx/stu
    • no support for direct page auto-magic
  • Zilog Z80
    • supports all undocumented opcodes
    • alternate mnemonics: exa for ex af,af', sls for sll, res/set (ix/iy+d)->r8 for res/set (ix/iy+d),r8, in (c) for in f,(c)
    • alternate register names: hx, lx, hy, ly for ixh, ixl, iyh, iyl

Schemta is written in CHICKEN Scheme, and can be used as stand-alone command line tool, or as a module in CHICKEN programs.

Rationale

The usual approach to writing assmblers in Lisp/Scheme is to model a set of procedures on the target CPU's instruction set, so that assembly source code must be rewritten as an s-expression. The expression is then evaluated to produce the resulting machine code.

Schemta takes a different approach. Assembly code for Schemta is regular assembly code, and Scheme provides the macro system of the assembler. This means that porting code written for other assemblers to Schemta and vice versa is easy, because syntax is largely compatible.

Schemta, Bintracker, and MDAL

Bintracker provides the Schemta API at runtime, so you can use it directly via the REPL or another live coding buffer.

MDAL uses Schemta as the assembler backend. When libmdal runs a Schemta assembly, it passes in the current MDAL module as the assembly-level symbol mdal_current_module. That means that assembly code used in an MDAL definition can reference any part of the module, including the MDAL definition.

Usage as a command-line tool

Schemta can also be compiled as a stand-alone application. The resulting executable can be invoked as follows:

$ schemta-exe [options]

With no arguments, schemta-exe will read from stdin and output to stdout.

The following options are available:

option effect
-i, --infile=FILENAME Compile the assembly source file FILENAME.
-o, --outfile=FILENAME Output to FILENAME. Defaults to {infile}.bin or stdout if reading from stdin.
--org=ADDRESS Specify the initial origin (compile address).
--cpu=CPU Specify the target CPU.
--equ=ALIST Additional symbol definitions to be passed in. ALIST is an associative list of key- value pairs wrapped in a string, eg. "((foo . 1) (bar . 2))".
-h, --help Print command line help and exit.

All arguments listed for long options are required for short options as well.

Usage

Syntax

Schemta mostly adheres to familiar syntax conventions established by other modern assemblers.

Comments

Any characters following a semi-colon ; until the end of the line are interpreted as a comment.

Instructions

Instructions must be preceded by at least one whitespace character. Each line may only contain one instruction. Mnemonics are case-insensitive.

Labels

Labels can contain any alphanumeric characters as well as underscores and the characters !?+-*/, but must start with a letter. Labels must not be indented.

Schemta also supports local labels. Local labels are symbols that can only be seen within the local namespace. The local namespace is limited by the non-local labels immediately preceding or following the local label declaration. Local labels must start with an underscore _. You can refer to a local label from outside the local namespace by prefixing the name of preceding non-local label to the identifier.

foo             ; a global label
  bra bar
  ;...
bar             ; another global label, opens local namespace "bar"
  bra _next
  ;...
_next           ; a local label
  bra baz
baz             ; another global label, closes preceding local namespace "bar"
  bra bar_next  ; referencing local label "_next" in the "_bar" namespace

Numbers

Schemta recognizes numeric arguments in binary, octal, decimal, and hexadecimal notation.

base prefix
binary %
decimal
octal 0o
hexadecimal $, 0x

Symbols

The same syntax rules as for labels apply.

Directives (Pseudo-Ops)

Schemta assembler directives are prefixed by a dot. A directive can be one of the pre-defined directives listed below, or a dot expression.

.align

Usage:

.align value [, fill]

Align the current origin to the next multiple of value, which must be a number, symbol, or dot expression. The optional fill argument specifies a byte value to be used as fill. If the second argument is omitted, memory will be filled with 0-bytes.

.cpu

Usage:

.cpu target-identifier

Set the target CPU. target-identifier must name one of the CPU targets supported by Schemta.

.db

Usage:

.db byte [, bytes...]

Insert one or more 8-bit values at the current origin. byte must be a number, single-quoted character, symbol, double-quoted string, or dot expression. Optionally, bytes... may be a comma-separated list of byte values. Double-quoted strings will be translated to a sequence of bytes representing ASCII values.

.dl

Usage:

.dl long [, longs...]

Insert one or more 32-bit values. long must be a number, symbol, or dot expression. Optionally, longs... may be a comma-separated list of long values.

.ds

Usage:

.ds amount [, fill]

Insert amount 8-bit values at the current origin. amount must be a number, symbol, or dot expression. The optional fill argument specifies a byte value to be used as fill. If the fill argument is omitted, memory will be filled with 0-bytes.

.dw

Usage:

.db word, [, words...]

Insert one or more 16-bit values. word must be a number, symbol, or dot expression. Optionally, words... may be a comma-separated list of word values.

.equ

Usage:

symbol .equ value

Define the symbol symbol, and set it's value to value. value may be a number, symbol, or dot expression.

.incbin

Usage:

.incbin filename

Include the contents of the binary file with the given filename.

.include

Usage:

.include filename

Include the contents of the assembly source file with the given filename.

.org

Usage:

.org address

Set the current origin (also known as compilation address) to address. address must be a number, symbol, or s-expression directive.

.pseudo-org

Usage:

.pseudo-org address

Keep assembling at the current origin, but treat the following code as if it were assembled at address. This is useful for blocks of code that will be copied to and executed at a different address at runtime. address must be a number, symbol, or s-expression directive.

Embedding Scheme with Dot-Expressions

Dot-Expressions are Schemta's version of an assembler macro system.

Any valid symbolic expression preceded by a dot is interpreted as Scheme code, provided the dot-expression combination does not represent one of the regular assembler directives. When using Schemta as a stand-alone application, it provides a Scheme interpreter that is mostly R5RS compliant, and additionally supports SRFIs 1, 13, and 14, and libmdal, as well as extensions to the R5RS standard provided by the CHICKEN implementation. When used from within Bintracker, Schemta has access to the entire Bintracker environment.

Most s-expressions valid in Scheme R5RS can be embedded, with the following exceptions:

  • Piped symbol names ('|foo bar|) are not allowed.
  • You cannot define any top-level bindings.
  • When the expression directive is used as an operand of an instruction, it must return a number.

You can, however, assign bindings to assembly level symbols. That means, while you cannot do this:

(define (foo arg) (body...))

this is perfectly fine:

foo .equ .(lambda (arg) (body...))

You can now use the procedure foo in your assembly code:

bar .equ .(foo arg)

  lda .(foo arg)

.if .(foo arg)
  .(asm ...)

  lda .(foo arg)

S-expression directives are normally evaluated directly. To return assembly code from an s-expression directive, the special procedure asm is provided, which takes a string as an argument. The string must contain any significant whitespace. Note that when you generate asm code, the generator expression may not refer to symbols that have not been defined yet. Forward references in the generated asm code are fine, of course.

asm-returning-proc .equ .(lambda () (asm "  lda #$10\n  tax\n"))

For accessing assembly level global and local symbols in Scheme, the symbol-ref and local-symbol-ref procedures are provided, which take a symbol identifier as the single argument. Note that the argument to these procedures may not be a procedure call.

.(if (= 0 (symbol-ref 'my-symbol))
     (asm "  lda #$1\n")
     (asm "  lda #$2\n"))

Conversely, you can define new assembly level symbols with the add-symbol! and add-local-symbol! procedures, which take a symbol identifier and a value as arguments.

The current origin address can be retrieved with the variable current-origin.

Returned assembly code can in turn contain s-expression directives, though the usefulness of this may be questionable.

Extending Schemta

You can add your own CPU targets in the target directory.

A target specification file should contain a single top-level s-expression which is laid out as follows:

(asm-target
 ;; KEYWORD            ;; ARGUMENT
 endian: little/big    ;; specify the endianness of the taret platform.
 registers: ()         ;; an alist where the keys are register names (as
                       ;; unquoted symbols), and values are substitution values
                       ;; that may be used for constructing output
                       ;; If there are any registers whose names are a substring
                       ;; of other register names, then the register with the
                       ;; longer name must be specified first.
 register-sets: ()     ;; an alist where the keys are arbitrary unique symbols,
                       ;; and values are lists of register names.
 addressing-modes: ()  ;; an alist where the keys are arbitrary unique symbols,
                       ;; and values are parser definitions (see below).
 flags: ()             ;; as registers, but for specifying condition flags
                       ;; only useful for instruction sets that use flags
                       ;; as arguments, rather than extensions to command names
 flag-sets: ()         ;; as register sets, but for flags
 extra: ()             ;; alist of additional functions/constants, usable from
                       ;; output generators
 instructions: ()      ;; an alist specifying the instruction set. See below.
  )

The instructions argument is mandatory. All other arguments are optional, provided you do not refer to any of them through the instruction specifiers.

Parsers

Schemta provides PEG parser combinators through comparse. In addition to the parsers provided by comparse, Schemta defines the following:

a-number

A plain number. Returns the numeric value.

numeric

Any of a-number, symbol, or a-sexp-directive.

sexp-directive

TODO...

symbol

An assembly level symbol. Returns `(symbol SYM).

(address ADDRESSING-MODE)

An address parser, defined in target-addressing-modes.

(flag CONDITION-SET)

A flag in FLAG-SET. FLAG-SET may be all for any flag in target-flags.

(register REGISTER-SET)

A register in REGISTER-SET. REGISTER-SET may be all for any register in target-registers.

Instructions

The instructions alist uses instruction names (as unquoted symbols) as keys. The values are in turn alists, where the keys are the possible number of arguments for the given command. For key 0, the value is a list representing the machine code that shall be output for that instruction. Otherwise, the value is an alist where the keys are parsers covering the first operand. The value is again either a list of machine code output, or an alist covering the second operand, and so forth.

Schemta internally converts all symbols, register and flag names to lowercase. This must be taken into account when writing instruction sets.

The elements of the output expression list must be byte values, or expressions evaluating to such a value. Within expressions, you may refer to the operands as %op1, %op2, etc. The following procedures are provided:

(lsb OPERAND)

(msb OPERAND)

Returns the least significant resp. most significant byte of a word-sized OPERAND value. If the operand is wider than 16 bits, msb returns the most significant byte of (bitwise-and operand #xffff).

(lsw OPERAND)

(msw OPERAND)

Returns the least significant resp. most significant word of the operand value.