OpenXM Stack machines

In OpenXM specification, all servers are stack machines.When a server ox_xyz gets an OX data message, it translates the data into a local object of ox_xyz and pushes the object onto the stack. The translation scheme is called the PhraseBook of ox_xyz (cf. OpenMath [1]). For example, the Asir command ox_push_cmo(P,1) (push integer $1$ onto the server $P$) sends an OX data message (OX_DATA,(CMO_ZZ,1)) to the server $P$. Here, OX_DATA stands for OX_DATA header and (CMO_ZZ,1) is a body standing for $1$ expressed by the CMO. The server translates (CMO_ZZ, 1) to its internal object of the integer $1$ and pushes the object onto the stack.

If the server gets an OX command message, then the server executes the command. Any OX command message starts with the int32 tag OX_COMMAND. The body is a stack machine operation code expressed by int32. The codes are listed below [12].

#define SM_popSerializedLocalObject               258
#define SM_popCMO                                 262
#define SM_popString                              263
#define SM_mathcap                                264
#define SM_pops                                   265
#define SM_setName                                266
#define SM_evalName                               267
#define SM_executeStringByLocalParser             268
#define SM_executeFunction                        269
#define SM_beginBlock                             270
#define SM_endBlock                               271
#define SM_shutdown                               272
#define SM_setMathCap                             273
#define SM_executeStringByLocalParserInBatchMode  274
#define SM_getsp                                  275
#define SM_dupErrors                              276
#define SM_control_kill                          1024
#define SM_control_to_debug_mode                 1025
#define SM_control_exit_debug_mode               1026
#define SM_control_reset_connection              1030

OpenXM does not have a standard for mathematical operation sets while it is a work in progress in the GAP group [7]. Each OpenXM server has its own set of mathematical operations, which are performed as follows. First, arguments for a mathematical operation and the number of the arguments are pushed. Second, the mathematical operator name, such as fctr (the factorization command of Asir), is pushed as a string. Finally, the stack machine command SM_executeFunction (269) evaluates the operator and pushes the result onto the stack after poping the operator name, the number of arguments and arguments. For example, the following code factorizes $x^{100}-1$ by calling ox_asir from Asir.

P = ox_launch(); 
ox_push_cmo(P,x^100-1); ox_push_cmo(P,ntoint32(1));
ox_push_cmo(P,"fctr");  ox_push_cmd(P,269); 
Ans = ox_pop_cmo(P);

When an error has occurred on an OpenXM server, an error object is pushed onto the stack instead of a result of the computation. The error object consists of the serial number of the OX message which caused the error, and an error message.

[340] P = ox_launch()$
[341] ox_rpc(P,"fctr",1.2*x)$
[342] ox_pop_cmo(P);
error([8,fctr : invalid argument])

OpenXM server won't send error messages to the client except when it receives a SM_pop* command. OX stack machines work in the asynchronous mode which is similar to X servers. For servers of graphic and sound applications, it may be an advantageous feature. It is also possible to emulate RPC and a web server for MCP [21] on our asynchronous OX stack machines.