Short overview of the sieve runtime machine

+
+1. Overview
+
+A compiled sieve program consists of a sequence of cells. Each cell
+is a pointer to sieve_op_t data, i.e. it points to an instruction handler
+(sieve_instr_t type) or to a memory location containing sieve_value_t
+structure. The exact interpretation of this pointer depends on the
+value of program counter. The very first entry in a sieve program
+(pc = 0) is always a NULL pointer. The next entry (pc = 1) is always
+expected to contain a valid instruction handler. When executing a
+program, the sieve runtime evaluator starts from pc = 1. It reads
+the cell contents, interprets it as address of an instruction handler,
+increments the program counter and executes the handler. The evaluator
+stops executing the program when it encounters a NULL pointer.
+
+When invoked, an instruction handler receives a single argument: a pointer
+to the sieve_machine_t structure, describing current runtime environment.
+If the handler needs any surplus arguments, it is its responsibility
+to retrieve them from the program cells immediately following the
+handler address and increment the pc value accordingly.
+
+2. Existing handlers
+
+2.1. Push
+
+Name: instr_push
+Arguments: None
+
+Pushes current numeric register on stack.
+
+2.2. Pop
+
+Name: instr_pop
+Arguments: None
+
+Pops the top of stack value into the numeric register.
+
+2.3. Unconditional branch
+
+Name: instr_branch
+Arguments: [pc  ]  (number) New value for pc.
+
+2.4. Branch if not zero
+
+Name: instr_brnz
+Arguments: [pc  ]  (number) New value for pc.
+
+2.5. Logical NOT
+
+Name: instr_not
+Arguments: none
+
+2.6.  Logical AND
+
+Name: instr_allof
+Arguments: [pc  ]  (number) Number of items to be popped from stack
+
+
+2.7.  Logical OR
+
+Name: instr_anyof
+Arguments: [pc  ]  (number) Number of items to be popped from stack
+
+2.8. Action handler
+
+Name: instr_action
+Arguments: [pc  ]  (sieve_handler_t*) Pointer to the action handling function.
+           [pc+1]  (list_t of sieve_value_t) A list of required arguments.  
+           [pc+2]  (list_t of sieve_runtime_tag_t) A list of tags.
+	   [pc+3]  (string) Name of the action (for debugging purposes).
+
+2.9. Test handler
+
+Name: instr_test
+Arguments: [pc  ]  (sieve_handler_t*) Pointer to the test handling function.
+           [pc+1]  (list_t of sieve_value_t) A list of required arguments.  
+           [pc+2]  (list_t of sieve_runtime_tag_t) A list of tags.
+	   [pc+3]  (string) Name of the test (for debugging purposes).
+
+3. Conditional statement branching
+
+A simple statement
+
+	IF cond list1 ELSE list2
+
+is translated into the following program
+
+# Compute the condition
+	.	
+	.
+	.
+	brnz	L_ELSE
+# Evaluate list1
+	.
+	.
+	.
+	br	L_END
+L_ELSE:
+# Evaluate list2
+	.
+	.
+	.
+L_END:
+	
+A more complex statement in the form:
+
+	IF cond1 list1 ELSIF cond2 list2 ELSE list3
+
+is translated into the following program
+
+# Compute the condition 1
+	.	
+	.
+	.
+	brnz	L_ELSIF
+# Evaluate list1
+	.
+	.
+	.
+	br	L_END
+L_ELSIF:
+# Compute the condition 2
+	.
+	.
+	.	
+	brnz	L_ELSE
+# Evaluate list 2
+	.
+	.
+	.
+	br	L_END
+L_ELSE:
+# Evaluate list 3
+	.
+	.
+	.
+L_END:
+
+Generally speaking, each ELSIF branch generates a code, similar to usual
+IF ... ELSE sequence.
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