This is geas – the Good Ethereum Assembler¹ – a macro assembler for the EVM.

You can use it to create any contract for Ethereum, though it's probably a bad idea. For real contracts, you should use a well-tested language compiler like Solidity. The purpose of geas is mostly creating specialty programs and tinkering with the EVM at a low level.

Installation

You can use the go tool to install the latest released version. This creates a geas binary in the current directory:

env "GOBIN=$PWD" go install github.com/fjl/geas@latest

For development of geas, clone the repository and then run go build ./cmd/geas.

Usage

To create bytecode from an assembly file, run the tool with a filename as argument.

./geas file.eas

There is also a disassembler. To disassemble hex bytecode from standard input, run:

./geas -d -

To see all supported flags, run geas with no arguments.

Editor Support

VIM users may be interested in vim-geas.

Use as a Go Library

You can also use the assembler as a library. See the API documentation to get started.

Language

Programs accepted by the assembler follow a simple structure. Each line is an instruction. Both uppercase and lowercase can be used for instruction names. All known EVM instructions are supported.

Comments can appear anywhere and are introduced by the semicolon (;) character.

    push 1  ;; comment
    push 2
    add

Opcodes listed in the program correspond directly with the bytecodes in output.

Jump

Jump destinations are written as a label followed by colon (:) and can be referred to using the notation @label together with JUMP or JUMPI.

begin:
    push 1
    push 2
    add
    jump @begin

When using JUMP with an argument, it turns into a PUSH of the label followed by the jump instruction, so the above is equivalent to:

begin:
    push 1
    push 2
    add
    push @begin
    jump

It is also possible to create labels without emitting a JUMPDEST instruction by prefixing the label name with the dot (.) character. While dotted labels are not valid for use as an argument to JUMP, they can be used with PUSH to measure code offsets.

    push @.end
    codesize
    eq
.end:

Push

The EVM instruction has sized push instructions from size zero (PUSH0) up to a size of 32 bytes (PUSH32). While you can use sized push instructions directly, it is preferable to let the assembler figure out the right size for you. To do this use the variable-size PUSH instruction.

All PUSH-type instructions must be followed by an immediate argument on the same line. Simple math expressions and label references can be used within the argument:

.begin:
    push (@add_it * 2) - 3
    push 5
add_it:
    add

Supported arithmetic operations include addition (+), subtraction (-), multiplication (*), division (/), and modulo (%). There is also support for bit-shifts (<<, >>), bitwise AND (&), OR (|), XOR (^).

All arithmetic is performed with arbitrary precision integers. The result of calculations must fit into 256 bits in order to be valid as a PUSH argument. For sized push, the result must fit into the declared push size. Negative results are not allowed.

Expression Macros

Expression macros can be created with the #define directive. Macros can be used within PUSH argument expressions.

Macros can have parameters. Refer to parameter values using the dollar sign ($) prefix within the macro.

#define z = 0x8823
#define myexpr(x, y) = ($x + $y) * z

    push myexpr(1, 2)

Builtin Macros

There are several builtin macros for common EVM tasks. Names of builtins start with a dot, and builtin macros cannot be redefined. Available builtins include:

.abs() for getting the absolute value of a number:

push .abs(0 - 100)

.selector() for computing 4-byte ABI selectors:

push .selector("transfer(address,uint256)")
push 0
mstore

.keccak256(), .sha256() hash functions:

push .sha256("data")

.address() for declaring contract addresses. The checksum and byte length of the address are verified.

#define otherContract = .address(0x658bdf435d810c91414ec09147daa6db62406379)

Instruction Macros

Common groups of instructions can be defined as instruction macros. Names of such macros always start with the percent (%) character.

#define %add5_and_store(x, location) {
    push $x
    push 5
    add
    push $location
    mstore
}

To invoke an instruction macro, write the macro name as a statement on its own line. If the macro has no arguments, you can also leave the parentheses off.

.begin:
    %add5_and_store(3, 64)
    %add5_and_store(4, 32)
    push 32
    push 64
    sha3

Nested macro definitions are not allowed. Macro recursion is also not allowed.

When defining (local) labels within instruction macros, they will only be visible within the macro. There is no way to refer to a local macro label from the outside, though you can pass references to such internal labels into another macro. The example below illustrates this, and also shows that in order to jump to a label argument within a macro, you must use explicit PUSH and JUMP.

#define %jump_if_not(label) {
    iszero
    push $label
    jumpi
}

#define %read_input(bytes) {
    calldatasize
    push $bytes
    eq
    %jump_if_not(@revert)

    push 0
    push $bytes
    calldataload
    jump @continue

  revert:
    push 0
    push 0
    revert

  continue:
}

Including Files

EVM assembly files can be included into the current program using the #include directive. Top-level instructions in the included file will be inserted at the position of the directive.

#include filenames are resolved relative to the file containing the directive.

.begin:
    push @.end
    push 32
    mstore

#include "file.evm"
.end:

Local and Global Scope

Names of labels and macros are case-sensitive. And just like in Go, the case of the first letter determines visibility of definitions.

Macro and label definitions whose name begins with a lower-case letter are local to the file they're defined in. This means local definitions cannot be referenced by #include files.

Identifiers beginning with an upper-case letter are registered in the global scope and are available for use across files. When using #include, global definitions in the included file also become available in all other files.

Global identifiers must be unique across the program, i.e. they can only be defined once. Files defining global macros or labels can only be included into the program once. Note that the uniqueness requirement also means that instruction macros containing global labels can only be called once. Use good judgement when structuring your includes to avoid redefinition errors.

lib.eas:

#define result = 128
#define StoreSum {
    add
    push result
    mstore
}

main.eas:

#include "lib.eas"

    push 1
    push 2
    %StoreSum  ;; calling global macro defined in lib.evm

Configuring the target instruction set

The EVM is a changing environment. Opcodes may be added (and sometimes removed) as new versions of the EVM are released in protocol forks. Geas is aware of EVM forks and their respective instruction sets.

Geas always operates on a specific EVM instruction set. It targets the latest known eth mainnet fork by default, i.e. all opcodes available in that fork can be used, and opcodes that have been removed in any prior fork cannot.

Use the #pragma target directive to change the target instruction set. The basic syntax is

#pragma target "name"

where name is a lower-case execution-layer fork name like homestead, berlin, or prague.

Here is an example. This contract uses the CHAINID instruction to check if it is running on mainnet, and destroys itself otherwise. CHAINID became available in the "istanbul" fork, and SELFDESTRUCT was removed in a later revision of the EVM, so this program is only applicable to a certain range of past EVM versions.

#pragma target "berlin"

    chainid                ; [id]
    push 1                 ; [1, id]
    eq                     ; [id = 1]
    jumpi @mainnet         ; []
    push 0x0               ; [zeroaddr]
    selfdestruct           ; []
mainnet:

Note that declaring the target instruction set using #pragma target will not prevent the output bytecode from running on a different EVM version, since it is just a compiler setting. The example program above will start behaving differently from its intended version on EVM version "cancun", because SELFDESTRUCT was turned into SENDALL in that fork. It may even stop working entirely in a later fork.

#pragma target can only appear in the program once. It cannot be placed in an include file. You have to put the directive in the main program file.

#assemble

When writing contract constructors and advanced CALL scenarios, it can be necessary to include subprogram bytecode as-is. The #assemble directive does this for you.

Using #assemble runs the assembler on the specified file, and includes the resulting bytecode into the current program. Labels of the subprogram will start at offset zero. Unlike with #include, global definitions of the subprogram are not imported.

    ;; copy subprogram to memory
    push @.end - @.begin   ; [size]
    push @.begin           ; [offset, size]
    push 128               ; [dest, offset, codesize]
    codecopy               ; []

.begin:
#assemble "subprogram.eas"
.end

If a target instruction set is configured with #pragma target, it will also be used for assembling the subprogram. However, the subprogram file can override the instruction set using its own #pragma target directive.

Footnotes

1. Under no circumstances must it be called the geth assembler. ↩