maquinas virtuales - virtual machines

[goosfancito]

Amigos,

Antonio en un momento me dio un ejemplo del uso de maquinas virtuales, para poder llamar funciones desde C pero que estan en harbour, ahora bien, como recupero un dato (cadena) que devuelve la funcion que llame y que esta en harbour.

Ejemplo:
function suma(a, b)
return ( a + b)

Como hago para que ese valor (a + b) lo pueda recuperar desde la funcion que fue llamada?
gracias

[Antonio Linares]

Gustavo,

hb_parc( -1 ) para cadenas
hb_parnl( -1 ) para números (sin decimales)
etc...

-1 accede al valor devuelto.

[goosfancito]

O sea de la misma forma que si recuperas un dato enviado a una funcion en C? pense que al utilizar Virtual la cosa cambiaba.

Gracias.

[Antonio Linares]

Si, es igual, ya que la máquina virtual usa un item en donde almacena el valor de retorno (lo que se devuelve con "return ...").

La clave está en que al usar -1 con las funciones del sistema extendido, se accede al dicho item que almacena el valor de retorno.

[leandro]

Perdon la ignoracia, pero alguien me puede explicar que son las maquinas virtuales?

[Antonio Linares]

Harbour y xharbour, al igual que Clipper, y al igual que otros compiladores (como Java, Visual Basic, etc.) usan lo que se denomina una máquina virtual.

La maquina virtual es como una CPU virtual. Es decir, no se ejecutan las instrucciones del microprocesador directamente, sino desde una capa de instrucciones controladas que aseguran la robustez de la aplicación. La aplicación nunca puede "desbocarse" como ocurre en lenguajes como C, ensamblador, etc.

La máquina virtual ofrece muchas ventajas para el desarrollo de aplicaciones de gestión y de todo tipo, pues aportan mucha ayuda para depurar las aplicaciones, es decir, para saber cuando ocurre un error y por qué.

La contrapartida es que las máquinas virtuales han de ser muy optimizadas para que no consuman demasiado tiempo en sus procesos, tiempo que es muy valioso para que la aplicación funcione rápido.

Aqui te incluyo un escrito que en su dia aporté al proyecto harbour, explicando la arquitectura de una máquina virtual:

The Harbour virtual machine (VM)

Question :

If a VM description is desirable, how should it be structured? (I
propose plain text, with two main sections: VM description, Opcodes. The
"Opcodes" section would describe every opcode: mnemonic, code, operands,
description. I think I can maintain this section.)

Answer :

The VM is formed by the main execution loop and several subsystems, each of
which could be theoretically replaced, supposing that you respect the
interface of each subsystem.

The main execution loop is defined in the C function named VirtualMachine(),
which receives two parameters: the pcode instructions to execute and the
local symbol table (a portion of the OBJs (static) symbol table) used by
that pcode: (please review hbpcode.h for the currently implemented pcode
opcodes)

VM( pcode, local symbols )

The VM may invoke the VM (itself) again. This lets the Clipper language
access Clipper functions and methods and external C language functions again
and again. The VM organizes these multiple accesses in an ordered and
fully controlled way, and implements services to access these multiple
execution levels (ProcName(), ProcLine(), debugging, and stack variables
access).

The VM subsystems are continuously used by the main execution loop. Let's
review these VM subsystems:

The startup: Controls the initialization of the different VM subsystems.
It is invoked at the beginning of the application. It also controls the
exiting of the application.

The stack: The VM does not use the stack of the computer directly, it uses
instead its own stack for manipulating values (parameters, returned values,
and symbols) as a hardware stack does.

The static symbol table: Created by the compiler at compile time (and
grouped by the linker on OBJs), this subsystem is responsible for an
immediate access to functions location and it is highly related to the
dynamic symbol table at runtime. It contains many duplicated symbols that
will be optimized by the dynamic symbol table.

The dynamic symbol table: Dynamically generated from the startup subsystem
at the beginning of the application. It organizes in an efficient way the
static symbol table creating an alphabetical index that allows a dicotomic
search of symbols. This subsystem is responsible for quick access to symbols
(functions, variables, fields and workareas aliases).

The static and public variables: Responsible for storing public and static
variables.

The memory: Responsible for allocating, reallocating, locking, unlocking and
freeing memory.

The extend system: Defines the interface (_parc(), ..., _retc() ) from low
level (C language) to high level (Clipper language). This subsystem is
responsible for connecting in a proper way C language functions to the
entire application.

Multidimensional arrays: This subsystem allows the creation of arrays, and
the services to manipulate these arrays in all ways. The arrays are
extensively used by the Clipper language and also they are the foundation of
the Objects (Objects are just arrays related to a specific Class).

The Objects engine: Responsible for the creation of Classes and Objects. It
also defines the way to access a specific Class method to be invoked by the
VM and provides all kind of Classes information that may be requested at
runtime.

The macro subsystem: it implements a reduced compiler that may be used at
runtime to generate pcode to be used by the application. In fact it is a
portion of the harbour yacc specifications.

The workareas subsystem: Responsible for databases management. It defines
the locations where the used workareas will be stored and provides all the
functions to access those workareas. It also implements the interface to the
replaceable database drivers.

Question :

Will Harbour opcodes mimic the Clipper ones? (will there be a 1:1
relation between them?) If so, are Clipper opcodes described somewhere?

Answer:

Clipper language pcode opcodes
DEFINE NAME VALOR BYTES
#define NOP 0x00 1
#define PUSHC 0x01 3 + literal
#define PUSHN 0x05 3
#define POPF 0x06 3
#define POPM 0x07 3
#define POPQF 0x08 3
#define PUSHA 0x09 3
#define PUSHF 0x0A 3
#define PUSHM 0x0B 3
#define PUSHMR 0x0C 3
#define PUSHP 0x0D 3
#define PUSHQF 0x0E 3
#define PUSHV 0x0F 3
#define SFRAME 0x10 3
#define SINIT 0x11 3
#define SYMBOL 0x12 3
#define SYMF 0x13 3
#define BEGIN_SEQ 0x19 3
#define JDBG 0x1A 3
#define JF 0x1B 3
#define JFPT 0x1C 3
#define JISW 0x1D 3
#define JMP 0x1E 3
#define JNEI 0x1F 3
#define JT 0x20 3
#define JTPF 0x21 3
#define PUSHBL 0x23 3
#define ARRAYATI 0x24 3
#define ARRAYPUTI 0x25 3
#define CALL 0x26 3
#define DO 0x27 3
#define FRAME 0x28 3
#define FUNC 0x29 3
#define LINE 0x2A 3
#define MAKEA 0x2B 3
#define MAKELA 0x2C 3
#define PARAMS 0x2D 3
#define POPFL 0x2E 3
#define POPL 0x2F 3
#define POPS 0x30 3
#define PRIVATES 0x31 3
#define PUBLICS 0x33 3
#define PUSHFL 0x34 3
#define PUSHFLR 0x35 3
#define PUSHI 0x36 3
#define PUSHL 0x37 3
#define PUSHLR 0x38 3
#define PUSHS 0x39 3
#define PUSHSR 0x3A 3
#define PUSHW 0x3B 3
#define SEND 0x3C 3
#define XBLOCK 0x3D 3
#define MPOPF 0x4A 5
#define MPOPM 0x4B 5
#define MPOPQF 0x4C 5
#define MPUSHA 0x4D 5
#define MPUSHF 0x4E 5
#define MPUSHM 0x4F 5
#define MPUSHMR 0x50 5
#define MPUSHP 0x51 5
#define MPUSHQF 0x52 5
#define MPUSHV 0x53 5
#define MSYMBOL 0x54 5
#define MSYMF 0x55 5
#define ABS 0x56 1
#define AND 0x57 1
#define ARRAYAT 0x58 1
#define ARRAYPUT 0x59 1
#define BREAK 0x5A 1
#define DEC 0x5B 1
#define DIVIDE 0x5C 1
#define DOOP 0x5D 1
#define EEQ 0x5E 1
#define ENDBLOCK 0x5F 1
#define ENDPROC 0x60 1
#define END_SEQ 0x61 1
#define EQ 0x62 1
#define EVENTS 0x63 1
#define FALSE 0x64 1
#define GE 0x65 1
#define GT 0x66 1
#define INC 0x67 1
#define LE 0x68 1
#define LT 0x69 1
#define MINUS 0x6A 1
#define MULT 0x6B 1
#define NE 0x6C 1
#define NEGATE 0x6E 1
#define NOP2 0x6F 1
#define NOT 0x70 1
#define NULL 0x71 1
#define ONE1 0x72 1
#define OR 0x73 1
#define PCOUNT 0x74 1
#define PLUS 0x75 1
#define POP 0x76 1
#define PUSHRV 0x77 1
#define QSELF 0x78 1
#define SAVE_RET 0x79 1
#define TRUE 0x7A 1
#define UNDEF 0x7B 1
#define ZER0 0x7C 1
#define ZZBLOCK 0x7D 1
#define AXPRIN 0x7E 1
#define AXPROUT 0x7F 1
#define BOF 0x80 1
#define DELETED 0x81 1
#define EOF 0x82 1
#define FCOUNT 0x83 1
#define FIELDNAME 0x84 1
#define FLOCK 0x85 1
#define FOUND 0x86 1
#define FSELECT0 0x87 1
#define FSELECT1 0x88 1
#define LASTREC 0x89 1
#define LOCK 0x8A 1
#define RECNO 0x8B 1
#define BNAMES 0x8C 1
#define LNAMES 0x8D 1
#define SNAMES 0x8E 1
#define SRCNAME 0x8F 1
#define TYPE 0x90 1
#define WAVE 0x91 1
#define WAVEA 0x92 1
#define WAVEF 0x93 1
#define WAVEL 0x94 1
#define WAVEP 0x95 1
#define WAVEPOP 0x96 1
#define WAVEPOPF 0x97 1
#define WAVEPOPQ 0x98 1
#define WAVEQ 0x99 1
#define WSYMBOL 0x9A 1
#define AADD 0x9B 1
#define ASC 0x9C 1
#define AT 0x9D 1
#define CDOW 0x9E 1
#define CHR 0x9F 1
#define CMONTH 0xA0 1
#define CTOD 0xA1 1
#define DATE 0xA2 1
#define DAY 0xA3 1
#define DOW 0xA4 1
#define DTOC 0xA5 1
#define DTOS 0xA6 1
#define EMPTY 0xA7 1
#define QEXP 0xA8 1
#define EXPON 0xA9 1
#define INSTR 0xAA 1
#define INT 0xAB 1
#define LEFT 0xAC 1
#define LEN 0xAD 1
#define LOGQ 0xAE 1
#define LOWER 0xAF 1
#define LTRIM 0xB0 1
#define MAX 0xB1 1
#define MIN 0xB2 1
#define MODULUS 0xB3 1
#define MONTH 0xB4 1
#define REPLICATE 0xB5 1
#define ROUND 0xB6 1
#define SECONDS 0xB7 1
#define SPACE 0xB8 1
#define QSQRT 0xB9 1
#define STR1 0xBA 1
#define STR2 0xBB 1
#define STR3 0xBC 1
#define SUB2 0xBD 1
#define SUB3 0xBE 1
#define TIME 0xBF 1
#define TRIM 0xC0 1
#define UPPER 0xC1 1
#define VAL 0xC2 1
#define VALTYPE 0xC3 1
#define WORD 0xC4 1
#define YEAR 0xC5 1
#define TRANS 0xC6 1
#define COL 0xC7 1
#define DEVPOS 0xC8 1
#define INKEY0 0xC9 1
#define INKEY1 0xCA 1
#define PCOL 0xCB 1
#define PROW 0xCC 1
#define ROW 0xCD 1
#define SETPOS 0xCE 1
#define SETPOSBS 0xCF 1

Harbour will not implement all of them as we want to provide the highest
freedom to programers to extend and modify Harbour as needed. In example:
Clipper language uses opcodes for: Row(), Col(), Upper(), Space(),
Replicate(), InKey(), Year(), Month(), etc... where we may just call a
standard C function, that uses the standard extend system and that may be
easily modified. So Harbour will use much less opcodes than the Clipper
language. This will also help to have a simpler and easier to maintain
compiler and VM.

Question :

I see that, for example, Harbour has an opcode named "PUSHWORD"(06),
while Valkyre calls it "PUSHW"(3B): Different names, different codes.
Isn't it desirable that Harbour pCode be binary-compatible with Clipper? I
mean, by doing so, Harbour VM could interpret Clipper pCode and
vice-versa.

Answer :

Harbour opcodes are defined in hbpcode.h. We are trying to use very easy to
remember mnemonics, so PUSHWORD seems easier than PUSHW. The opcodes values
are meaningless as they are just used by a C language switch sentence (in
fact there is a powerful optimization which is to use the pcode opcodes
themselves as an index to a VM functions pointers array, so VM execution
speed may increase. Clipper uses it).

We are not fully implementing the Clipper language OBJs model (i.e. to
provide identifiers names length higher than 10 chars) so Harbour OBJs will not be supported by Clipper and viceversa.