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diff --git a/spec.org b/spec.org new file mode 100644 index 0000000..8878161 --- /dev/null +++ b/spec.org @@ -0,0 +1,46 @@ +#+title: VM Specification +#+author: Aryadev Chavali +#+description: A specification of instructions for the virtual machine +#+date: 2023-11-02 + +* Data types +There are 3 main data types of the virtual machine. They are all +unsigned. There exist signed versions of these data types, though +there is no difference in terms of bytecode between them. For an +unsigned type <T> the signed version is simply S_<T>. +|-------+------| +| Name | Bits | +|-------+------| +| Byte | 8 | +| HWord | 32 | +| Word | 64 | +|-------+------| +* Instructions +An instruction for the virtual machine is composed of an *opcode* and, +potentially, an *operand*. An /opcode/ represents the behaviour of +the instruction i.e. what _is_ the instruction. The /operand/ is a +datum of one of the /data types/ described previously. + +Some instructions do have /operands/ while others do not. The former +type of instructions are called *UNIT* instructions while the latter +type are called *MULTI* instructions[fn:1]. + +All /opcodes/ (with very few exceptions[fn:2]) have two components: +the *root* and the *type specifier*. The /root/ represents the +general behaviour of the instruction: ~PUSH~, ~POP~, ~MOV~, etc. The +/type specifier/ specifies what /data type/ it manipulates. A +complete opcode will be a combination of these two e.g. ~PUSH_BYTE~, +~POP_WORD~, etc. Some /opcodes/ may have more /type specifiers/ than +others. +* Bytecode format +Bytecode files are byte sequence which encode instructions for the +virtual machine. Any instruction (even with an operand) has one and +only one byte sequence associated with it. +* Footnotes +[fn:2] ~NOOP~, ~HALT~, ~MDELETE~, ~MSIZE~, ~JUMP_*~ +[fn:1] /UNIT/ refers to the fact that the internal representation of +these instructions are singular: two instances of the same /UNIT/ +instruction will be identical in terms of their binary. On the other +hand, two instances of the same /MULTI/ instruction may not be +equivalent due to the operand they take. Crucially, most if not all +/MULTI/ instructions have different versions for each /data type/. |