atomontage / hexaforth

Experiments in unencoded instructions for a Forth VM

hexaforth

Work-in-progress experiments in unencoded instructions for performant stack machines; incomplete.

Why?

Traditionally, many p-code VMs are implemented as bytecode. I wanted to investigate how encoded instructions such as implemented by Novix NC4016 or the J1 Forth CPU used perform on modern architectures with a VM.

What?

The instruction encoding draws heavily from the J1b Forth CPU described at swapforth/j1b/basewords.fs. There are some notable differences, as I wanted to implement a 64-bit VM rather than a 16-bit VM, specifically how literals are handled.

Literal Handling

        struct {
            bool  lit_f: 1;
            bool  lit_add: 1;       // add
            BYTE  lit_shifts: 2;    // 0-3 shifts
            WORD  lit_v: 12;
        } lit;

If the first bit of the instruciton is on, it's a literal. The literal is a 12-bit unsigned number, in lit_v. Of particular interest is the lit_shifts field, which tells the VM to shift the lit_v number left by lit_shifts * 12 bits. This allows us to express 48 bits of magnitude in a 12-bit literal instruction.

The lit_add flag determines if the literal should be added to T, the top element of the stack, and has the following characteristics:

• true -- add shifted lit_v value to T, and do not advance the SP; we are working with the existing value in place.
• false -- advance the SP and set the new T to the shifted lit_v value.

An astute reader might have noted that the stack is 64-bit signed, but that our literals are unsigned. This is taken care of by the literal encoder, inserting an instruction to invert the number on the stack.

A bonus characteristic of this literal implementation is that the literal instruction can be used as an in-line increment instruction for numbers up to 4096, without having to push the value onto the stack, doing in one VM cycle what would normally take three.

Some examples of the literal encoding, such as for -20. As it is less than 4096, we encode it as a single literal, and then we negate it with the following invert opcode call.

COMPILE_LITERAL: -20
HERE[0]: 0x0141 => lit: {lit_f=1, lit_add=0, shifts=0, lit.lit_v=20 => 20}
HERE[1]: 0x0036 => alu: {alu_op: 3, op: ~T, DSTACK: 0 RSTACK: 0, N->[T]: 0} => invert

An example of how we leverage the lit_add field is the number -24923. It's turned into an unsigned 24923, the rightmost 12-bits are encoded as 347, and then the next 12-bits are encoded as 24576 with the lit_add flag set to 1. Then we invert.

COMPILE_LITERAL: -24923
HERE[11]: 0x15b1 => lit: {lit_f=1, lit_add=0, shifts=0, lit.lit_v=347 => 347}
HERE[12]: 0x0067 => lit: {lit_f=1, lit_add=1, shifts=1, lit.lit_v=6 => 24576}
HERE[13]: 0x0036 => alu: {alu_op: 3, op: ~T, DSTACK: 0 RSTACK: 0, N->[T]: 0} => invert

The literal compiler is smart enough not to bother encoding 12-bit segments that are 0, skipping over them such as for 32768. This makes the literal encoding perfect for memory offset calculations up to 48-bits.

COMPILE_LITERAL: 32768
HERE[14]: Skipping LIT instruction as lit_v == 0, shift=0
HERE[14]: 0x0085 => lit: {lit_f=1, lit_add=0, shifts=1, lit.lit_v=8 => 32768}

ALU Instructions

ALU instrucitons are roughly divided into two sections. When op_type is OP_TYPE_ALU, we use alu_op for the actual operation and mem_op for 'what to do with the result of the ALU op'. We also have fields for whether we should adjust the data stack or return stack, and whether we should write to memory.

        struct {
            bool    lit_f: 1;
            BYTE    op_type: 2;
            BYTE    alu_op: 4;
            BYTE    mem_op: 3;
            bool    __unused: 1;
            BYTE    dstack: 2;
            BYTE    rstack: 2;
            bool    ram_write: 1;
        } alu;

We have the following alu_op:

ALU Op	Op	Descripton
T	0x0	Do something with T
N	0x1	Set T to N
T+N	0x2	Add T and N
T&N	0x3	Bitwise AND of T and N
`T	N`	0x4
T^N	0x5	Bitwise XOR of T and N
~T	0x6	Bitwise inversion ofT
T==N	0x7	Test if T and N are equivalent
T>N	0x8	Test if T is greater than N
N>>T	0x9	Bitshift N left by T bits.
N<<T	0xA	Bitshift N right by T bits.
R->T	0xB	Copy the top of return stack R to T
[T]	0xC	Load address located at T into T.
io[T]	0xD	IO read at T.
depth	0xE	Depth of the stack
T>Nu	0xF	Unsigned test for if T is greater than N

Then we have the following mem_ops:

Mem Op	Op	Descripton
T->N	0x0	Copy T into N
T->R	0x1	Copy T into R, the top of the return stack
N->[T]	0x2	Store N into address T
io[N]->T	0x3	Write IO operation Ninto T
io@	0x4	Read IO operation into T
R->EIP	0x5	Copy the R stack into the IP pointer

Then with dstack and rstack, we can adjust the size of the data and return stacks.

Finally, we have ram_write, which is to write N into the address at T.

Effectively, this encoding allows us to have microinstructions that we construct Forth words out of.

An example is ! which is 'write N to [T]', encoded as such:

        {"!", {
                        { .alu.op_type = OP_TYPE_ALU,
                          .alu.alu_op = ALU_N,
                          .alu.dstack = -1,
                          .alu.ram_write = 1 }}},