Condition code flag emulation for program code conversion

Abstract

An emulator ( 30 ) allows subject code ( 10 ) written for a subject processor ( 12 ) having subject processor registers ( 14 ) and condition code flags ( 16 ) to run in a non-compatible computing environment ( 2 ). The emulator ( 3 ) identifies and records parameters of instructions in the subject code ( 10 ) that affect status of the subject condition code flags ( 16 ). Then, when an instruction in the subject code ( 10 ) is encountered, such as a branch or jump, that uses the flag status to make a decision, the flag status is resolved from the recorded instruction parameters. Advantageously, emulation overhead is substantially reduced.

1 . A method for emulating condition code flags during program code conversion, comprising the steps of: 
 (a) identifying a flag-affecting instruction in a sequence of subject code instructions;    (b) recording parameters of the identified flag-affecting instruction;    (c) detecting a flag-usage instruction in the sequence of subject code instructions; and    (d) resolving a flag status with respect to the recorded parameters.    
     
     
         2 . The method of  claim 1 , wherein the step (d) comprises explicitly calculating a flag status by emulating effects of the identified flag-affecting instruction on one or more subject condition code flags.  
     
     
         3 . The method of  claim 2 , wherein the step (b) comprises setting a flag status indicator to a pending state to indicate that parameters have been recorded, and the step (d) comprises setting the flag status indicator to a normalised state to indicate that a flag status has been explicitly calculated.  
     
     
         4 . The method of  claim 1 , wherein the step (d) comprises implicitly determining flag status with reference to the recorded parameters.  
     
     
         5 . The method of  claim 1 , wherein the step (c) comprises determining a flag usage type with reference to a type of the detected flag-usage instruction, and the step (d) comprises resolving flag status either by explicitly calculating a flag status or by implicitly determining a flag status, according to the determined flag usage type.  
     
     
         6 . The method of  claim 1 , wherein the step (a) comprises identifying a second flag-identifying instruction in the sequence of subject code instructions, and the step (b) comprises replacing the recorded parameters with parameters of the second flag-affecting instruction.  
     
     
         7 . The method of  claim 1 , wherein the step (a) comprises identifying a type of flag-affecting instruction as one of a plurality of predetermined types, and the step (b) comprises recording parameters for each type of flag-affecting instruction.  
     
     
         8 . The method of  claim 7 , wherein the predetermined types include a first type instruction affecting a full set of condition code flags, and a second type instruction affecting a subset of the full set of condition code flags.  
     
     
         9 . The method of  claim 8 , wherein the step (d) comprises resolving flag status for a full set of condition code flags with respect to the recorded first-type parameters, and resolving flag status with respect to the subset of the condition code flags with respect to the recorded second-type parameters.  
     
     
         10 . The method of  claim 9 , wherein the step (d) comprises explicitly determining flag status from the recorded first-type parameters, and modifying the explicitly determined flag status from the- recorded second-type parameters.  
     
     
         11 . The method of  claim 7 , wherein the predetermined types include instruction types each affecting a condition code flag set amongst a plurality of condition code flag sets.  
     
     
         12 . The method of  claim 1 , wherein the recorded parameters include an instruction type parameter and one or more instruction operand parameters.  
     
     
         13 . The method of  claim 12 , wherein recording an instruction operand parameter comprises storing a reference to a location containing an operand.  
     
     
         14 . The method of  claim 12 , wherein recording an instruction operand parameter comprises determining that an operand remains available unmodified at an original location and storing a reference to the original location as the instruction operand parameter, or else copying the operand from the original location to a dedicated operand parameter storage location.  
     
     
         15 . The method of  claim 12 , comprising providing a plurality of abstract registers representing registers of a subject processor, and supplementing the abstract registers with one or more additional abstract registers each for storing an operand as an instruction operand parameter.  
     
     
         16 . A method for emulating condition code flags during program code conversion, comprising: 
 selecting a sequence of subject code instructions;    identifying a flag-affecting instruction in the sequence of subject code instructions, the flag-affecting instruction specifying an operation that affects subject condition code flags of a subject processor;    recording parameters of the identified flag-affecting instruction including an instruction type parameter and one or more operand parameters;    detecting a flag-usage instruction in the sequence of subject code instructions, the flag-usage instruction specifying an operation with reference to a flag status of one or more of the subject condition code flags; and    in response to detecting a flag-usage instruction, resolving a flag status of one or more of the subject condition code flags with respect to the recorded parameters.    
     
     
         17 . The method of  claim 16 , comprising updating the recorded parameters when a new flag-affecting instruction is encountered in the sequence of subject code instructions.  
     
     
         18 . The method of  claim 16 , comprising recording parameters for a plurality of flag-affecting instructions.  
     
     
         19 . The method of  claim 18 , wherein the plurality of flag-affecting instructions are each associated with differing sets of subject condition code flags.  
     
     
         20 . The method of  claim 16 , comprising resolving flag status by at least one of (a) explicitly calculating a status for one or more subject condition code flags by emulating an effect of the recorded flag-affecting instruction, or (b) implicitly representing the flag-usage instruction using the recorded parameters, or (c) selecting between options (a) and (b) according to a type of the flag-usage instruction.  
     
     
         21 . The method of  claim 16 , wherein the recorded parameters include at least one instruction operand parameter, and wherein recording the instruction operand parameter comprises .at least one of (a) copying an operand to a predetermined storage location, or (b) storing a reference to an original location containing the operand, or (c) selectively performing (a) or (b).  
     
     
         22 . An emulator apparatus for use in a target computing environment for emulating a subject processor of a subject computing environment when translating subject code appropriate to the subject computing environment to produce target code appropriate to the target computing environment, the emulator apparatus comprising: 
 means for identifying a flag-affecting instruction in a sequence of subject code instructions;    an instruction parameter store for recording parameters of the identified flag-affecting instruction;    means for detecting a flag-usage instruction in the sequence of subject code instructions; and    means for resolving a flag status with respect to the instruction parameters recorded in the instruction parameter store.    
     
     
         23 . A computing platform comprising the emulator apparatus of  claim 22 .  
     
     
         24 . A computer-readable recording medium containing program code constructions for performing the method of  claim 1 .  
     
     
         25 . A computer-readable recording medium containing program code constructions for performing the method of  claim 16.