Erlang BEAM memo

beam.rst

BEAM

assemble

$ erlc -S test.erl

registers

• 1024 X registers
• Y register

Registers X, Y are represented like this:

5 bit tag

27 bit value

The tags are 3=X, 4=Y, 5=r, 6=Literal

• CP

• "Continuation Pointer" - return address

• PC

• "Program Counter" - next instruction to be executed

important instructions

• test
• move {move, Src, Dest}

64 is the opcode for "move", 0x18000000 is the encoding of argument that means register {x, 0}

• gc_bif
• call {call, Arity, {f, N}}

• set P=N, CP=Next
• jumps to label N
• arguments are stored in X0 ~ Xn

• return

• does PC=CP
• return value is stored in X0

• receive
• allocate
• deallocate {deallocate, N}

• create and remove stack frames.
• there also exists {allocate_zero, Ny, Ng} - Ny Y registers

other instructions

• label {label, N}

• Nth label in the module

• function {function, Name, Arity, LabelID}

• defines a function

• bif {bif, FuncAtom, {f, 0}, [], {x, 0}}

• calls, ?

• gc_bif {gc_bif, '*', {f, A}, N1, [R1, R2, R3]}

• jumps to a function named '*'
• multiply R1 and R2, and set the result into R3. If any failure occurs jump to A. If GC is triggered include N1 X registers starting at X0 in the root set for GC.

• send send.

• sends the data in X0 ?

• loop_rec {loop_rec, {f, N}, {x, 0}}

• receive clause starts here, till loop_rec_end

• wait_timeout {wait_timeout, {f, N}, {integer, T}}
• test {test, is_eq_exact, {f, N}, [{x,0}, {atom, foo}]}
• remove_message.

misc

Context Switch

No other timing than when a call or receive instruction is executed, check a reduction counter.

How to disasemble

beam_lib:chunks("hw.beam", [abstract_code]),

Reference

• instruction set? genop.tab
• source code - beam_emu.c
• erlang vm opcodes: beam_opcodes.erl
• Joe Armsgrong, Deconstructing the BEAM , 2012.
• The Erlang BEAM Virtual Machine Specification
• File format for Beam R5 and later
• The Evolution of the Erlang VM

- A Guide to the Erlang Source ― Erlang source code guide