General Computer Science 320201 GenCS I & II Lecture ... - Kwarc

Definition 457 An ASM program VM is called a virtual machine for L(VM), iff VM inputs a
L(VM) program (as data) and runs it on REMA.
Plan: Instead of building a compiler for SML to ASM, build a virtual machine VM for REMA
and a compiler from SML to L(VM). (simpler and more transparent)
c○: Michael Kohlhase 281
The main difference between the register machine REMA and the virtual machine VM construct
is the way it organizes its memory. The REMA gives the assembler language full access to its
internal registers and the data store, which is convenient for direct programming, but not suitable
for a language that is mainly intended as a compilation target for higher-level languages which
have regular (tree-like) structures. The virtual machine VM builds on the realization that tree-like
structures are best supported by stack-like memory organization.
A Virtual Machine for Functional Programming
We will build a stack-based virtual machine; this will have four components
Stack Program Store
Command Interpreter
VPC
The stack is a memory segment operated as a “last-in-first-out” LIFO sequence
The program store is a memory segment interpreted as a sequence of instructions
The command interpreter is a ASM program that interprets commands from the program
store and operates on the stack.
The virtual program counter (VPC) is a register that acts as a the pointer to the current
instruction in the program store.
The virtual machine starts with the empty stack and VPC at the beginning of the program.
A Stack-based Programming Language
c○: Michael Kohlhase 282
Now we are in a situation, where we can introduce a programming language for VM. The main
difference to ASM is that the commands obtain their arguments by popping them from the stack
(as opposed to the accumulator or the ASM instructions) and return them by pushing them to the
stack (as opposed to just leaving them in the registers).
A Stack-Based VM language (Arithmetic Commands)
Definition 458 VM Arithmetic Commands act on the stack
instruction effect VPC
con i pushes i onto stack VPC: = VPC + 2
add pop x, pop y, push x + y VPC: = VPC + 1
sub pop x, pop y, push x − y VPC: = VPC + 1
mul pop x, pop y, push x · y VPC: = VPC + 1
leq pop x, pop y, if x ≤ y push 1, else push 0 VPC: = VPC + 1
Example 459 The L(VM) program “con 4 con 7 add” pushes 7 + 4 = 11 to the stack.
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