Guile VM Specification -*- outline -*- ====================== Updated: $Date: 2000/08/22 15:54:19 $ * Introduction A Guile VM has a set of registers and its own stack memory. Guile may have more than one VM's. Each VM may execute at most one program at a time. Guile VM is a CISC system so designed as to execute Scheme and other languages efficiently. ** Registers pc - Program counter ;; ip (instruction poiner) is better? sp - Stack pointer bp - Base pointer ac - Accumulator ** Engine A VM may have one of three engines: reckless, regular, or debugging. Reckless engine is fastest but dangerous. Regular engine is normally fail-safe and reasonably fast. Debugging engine is safest and functional but very slow. ** Memory Stack is the only memory that each VM owns. The other memory is shared memory that is shared among every VM and other part of Guile. ** Program A VM program consists of a bytecode that is executed and an environment in which execution is done. Each program is allocated in the shared memory and may be executed by any VM. A program may call other programs within a VM. ** Instruction Guile VM has dozens of system instructions and (possibly) hundreds of functional instructions. Some Scheme procedures such as cons and car are implemented as VM's builtin functions, which are very efficient. Other procedures defined outside of the VM are also considered as VM's functional features, since they do not change the state of VM. Procedures defined within the VM are called subprograms. Most instructions deal with the accumulator (ac). The VM stores all results from functions in ac, instead of pushing them into the stack. I'm not sure whether this is a good thing or not. * Variable Management A program may have access to local variables, external variables, and top-level variables. ** Local/external variables A stack is logically divided into several blocks during execution. A "block" is such a unit that maintains local variables and dynamic chain. A "frame" is an upper level unit that maintains subprogram calls. Stack dynamic | | | | chain +==========+ - = | |local vars| | | `-|block data| | block | /|frame data| | | | +----------+ - | | |local vars| | | frame `-|block data| | | /+----------+ - | | |local vars| | | `-|block data| | | /+==========+ - = | |local vars| | | `-|block data| | | /|frame data| | | | +----------+ - | | | | | | The first block of each frame may look like this: Address Data ------- ---- xxx0028 Local variable 2 xxx0024 Local variable 1 bp ->xxx0020 Local variable 0 xxx001c Local link (block data) xxx0018 External link (block data) xxx0014 Stack pointer (block data) xxx0010 Return address (frame data) xxx000c Parent program (frame data) The base pointer (bp) always points to the lowest address of local variables of the recent block. Local variables are referred as "bp[n]". The local link field has a pointer to the dynamic parent of the block. The parent's variables are referred as "bp[-1][n]", and grandparent's are "bp[-1][-1][n]". Thus, any local variable is represented by its depth and offset from the current bp. A variable may be "external", which is allocated in the shared memory. The external link field of a block has a pointer to such a variable set, which I call "fragment" (what should I call?). A fragment has a set of variables and its own chain. local external chain| | chain | +-----+ .--------, | `-|block|--+->|fragment|-' /+-----+ | `--------'\, `-|block|--' | /+-----+ .--------, | `-|block|---->|fragment|-' +-----+ `--------' | | An external variable is referred as "bp[-2]->variables[n]" or "bp[-2]->link->...->variables[n]". This is also represented by a pair of depth and offset. At any point of execution, the value of bp determines the current local link and external link, and thus the current environment of a program. Other data fields are described later. ** Top-level variables Guile VM uses the same top-level variables as the regular Guile. A program may have direct access to vcells. Currently this is done by calling scm_intern0, but a program is possible to have any top-level environment defined by the current module. *** Scheme and VM variable Let's think about the following Scheme code as an example: (define (foo a) (lambda (b) (list foo a b))) In the lambda expression, "foo" is a top-level variable, "a" is an external variable, and "b" is a local variable. When a VM executes foo, it allocates a block for "a". Since "a" may be externally referred from the closure, the VM creates a fragment with a copy of "a" in it. When the VM evaluates the lambda expression, it creates a subprogram (closure), associating the fragment with the subprogram as its external environment. When the closure is executed, its environment will look like this: block Top-level: foo +-------------+ |local var: b | fragment +-------------+ .-----------, |external link|---->|variable: a| +-------------+ `-----------' The fragment remains as long as the closure exists. ** Addressing mode Guile VM has five addressing modes: o Real address o Local position o External position o Top-level location o Immediate object Real address points to the address in the real program and is only used with the program counter (pc). Local position and external position are represented as a pair of depth and offset from bp, as described above. These are base relative addresses, and the real address may vary during execution. Top-level location is represented as a Guile's vcell. This location is determined at loading time, so the use of this address is efficient. Immediate object is not an address but gives an instruction an Scheme object directly. [ We'll also need dynamic scope addressing to support Emacs Lisp? ] *** At a Glance Guile VM has a set of instructions for each instruction family. `%load' is, for example, a family to load an object from memory and set the accumulator (ac). There are four basic `%load' instructions: %loadl - Local addressing %loade - External addressing %loadt - Top-level addressing %loadi - Immediate addressing A possible program code may look like this: %loadl (0 . 1) ; ac = local[0][1] %loade (2 . 3) ; ac = external[2][3] %loadt (foo . #) ; ac = # %loadi "hello" ; ac = "hello" One instruction that uses real addressing is `%jump', which changes the value of the program counter: %jump 0x80234ab8 ; pc = 0x80234ab8 * Program Execution Overall procedure: 1. A source program is compiled into a bytecode. 2. A bytecode is given an environment and becomes a program. 3. A VM starts execution, creating a frame for it. 4. Whenever a program calls a subprogram, a new frame is created for it. 5. When a program finishes execution, it returns a value, and the VM continues execution of the parent program. 6. When all programs terminated, the VM returns the final value and stops. ** Environment Local variable: (let ((a 1) (b 2) (c 3)) (+ a b c)) -> %pushi 1 ; a %pushi 2 ; b %pushi 3 ; c %bind 3 ; create local bindings %pushl (0 . 0) ; local variable a %pushl (0 . 1) ; local variable b %pushl (0 . 2) ; local variable c add 3 ; ac = a + b + c %unbind ; remove local bindings External variable: (define foo (let ((n 0)) (lambda () n))) %pushi 0 ; n %bind 1 ; create local bindings %export [0] ; make it an external variable %make-program # ; create a program in this environment %unbind ; remove local bindings %savet (foo . #) ; save the program in foo (foo) -> %loadt (foo . #) ; program has an external link %call 0 ; change the current external link %loade (0 . 0) ; external variable n %return ; recover the external link Top-level variable: foo -> %loadt (foo . #) ; top-level variable foo ** Flow control (if #t 1 0) -> %loadi #t %br-if-not L1 %loadi 1 %jump L2 L1: %loadi 0 L2: ** Function call Builtin function: (1+ 2) -> %loadi 2 ; ac = 2 1+ ; one argument (+ 1 2) -> %pushi 1 ; 1 -> stack %loadi 2 ; ac = 2 add2 ; two argument (+ 1 2 3) -> %pushi 1 ; 1 -> stack %pushi 2 ; 2 -> stack %pushi 3 ; 3 -> stack add 3 ; many argument External function: (version) -> %func0 (version . #) ; no argument (display "hello") -> %loadi "hello" %func1 (display . #) ; one argument (open-file "file" "w") -> %pushi "file" %loadi "w" %func2 (open-file . #) ; two arguments (equal 1 2 3) %pushi 1 %pushi 2 %pushi 3 %loadi 3 ; the number of arguments %func (equal . #) ; many arguments ** Subprogram call (define (plus a b) (+ a b)) (plus 1 2) -> %pushi 1 ; argument 1 %pushi 2 ; argument 2 %loadt (plus . #) ; load the program %call 2 ; call it with two arguments %pushl (0 . 0) ; argument 1 %loadl (0 . 1) ; argument 2 add2 ; ac = 1 + 2 %return ; result is 3 * Instruction Set The Guile VM instruction set is roughly divided two groups: system instructions and functional instructions. System instructions control the execution of programs, while functional instructions provide many useful calculations. By convention, system instructions begin with a letter `%'. ** Environment control instructions - %alloc - %bind - %export - %unbind ** Subprogram control instructions - %make-program - %call - %return ** Data control instructinos - %push - %pushi - %pushl, %pushl:0:0, %pushl:0:1, %pushl:0:2, %pushl:0:3 - %pushe, %pushe:0:0, %pushe:0:1, %pushe:0:2, %pushe:0:3 - %pusht - %loadi - %loadl, %loadl:0:0, %loadl:0:1, %loadl:0:2, %loadl:0:3 - %loade, %loade:0:0, %loade:0:1, %loade:0:2, %loade:0:3 - %loadt - %savei - %savel, %savel:0:0, %savel:0:1, %savel:0:2, %savel:0:3 - %savee, %savee:0:0, %savee:0:1, %savee:0:2, %savee:0:3 - %savet ** Flow control instructions - %br-if - %br-if-not - %jump ** Function call instructions - %func, %func0, %func1, %func2 ** Scheme buitin functions - cons - car - cdr ** Mathematical buitin functions - 1+ - 1- - add, add2 - sub, sub2, minus - mul2 - div2 - lt2 - gt2 - le2 - ge2 - num-eq2