# zkas bincode

The bincode design for zkas is the compiled code in the form of a binary blob, that can be read by a program and fed into the VM.

Our programs consist of three sections: constant, contract, and circuit. Our bincode represents the same. Additionally, there is an optional section called .debug which can hold debug info related to the binary.

We currently keep everything on the same stack, so we avoid having to deal with different types. Instead, we rely that the compiler does a proper parse and analysis of the source code, so we are sure that in the VM, when referenced, the types shall be correct.

The compiled binary blob has the following layout:

MAGIC_BYTES
BINARY_VERSION
.constant
CONSTANT_TYPE CONSTANT_NAME
CONSTANT_TYPE CONSTANT_NAME
...
.contract
WITNESS_TYPE
WITNESS_TYPE
...
.circuit
OPCODE ARG_NUM STACK_INDEX ... STACK_INDEX
OPCODE ARG_NUM STACK_INDEX ... STACK_INDEX
...
.debug
TBD


Integers in the binary are encoded using variable-integer encoding. See serial.rs for our Rust implementation.

## MAGIC_BYTES

The magic bytes are the file signature consisting of four bytes used to identify the zkas binary code. They consist of:

0x0b 0xxx 0xb1 0x35

## BINARY_VERSION

The binary code also contains the binary version to allow parsing potential different formats in the future.

0x01

## .constant

The constants in the .constant section are declared with their type and name, so that the VM knows how to search for the builtin constant and add it to the stack.

## .contract

The .contract section holds the circuit witness values in the form of WITNESS_TYPE. Their stack index is incremented for each witness as they're kept in order like in the source file. The witnesses that are of the same type as the circuit itself (typically Base) will be loaded into the circuit as private values using the Halo2 load_private API.

## .circuit

The .circuit section holds the procedural logic of the ZK proof. In here we have statements with opcodes that are executed as understood by the VM. The statements are in the form of:

OPCODE ARG_NUM STACK_INDEX ... STACK_INDEX

where:

ElementDescription
OPCODEThe opcode we wish to execute
ARG_NUMThe number of arguments given to this opcode
(Note the VM should be checking the correctness of this as well)
STACK_INDEXThe location of the argument on the stack.
(This is supposed to be repeated ARG_NUM times)

In case an opcode has a return value, the value shall be pushed to the stack and become available for later references.

TBD

## Decoding the bincode

An example decoder implementation can be found in zkas' decoder.rs module.