Apparatus and method for correcting microcomputer software errors

An apparatus for replacing an undesired code stored in a read-only memory (ROM) with a replacement code, comprising:

• an electronically programmable memory having a first storage location for storing a beginning address of the undesired code, a second storage location for storing a beginning address of the replacement code, and a plurality of addressable storage locations for storing the replacement code wherein the last of said addressable storage locations stores an instruction for readdressing said ROM;
• addressing means for addressing both said ROM and said electronically programmable memory;
• transfer means coupled to said first and second memory locations and to said addressing means for transferring said beginning address of the replacement code to said addressing means when said beginning address of said undesired code is generated by said addressing means;
• and
• a central processing unit receiving code from said ROM and said electronically programmable memory, said central processing unit instructing said addressing means to address said ROM when said instruction for readdressing said ROM is recognized.

An apparatus for replacing an undesired code stored in a plurality of storage locations in read-only memory (ROM) with a replacement code, comprising:

• an electronically programmable memory having a first storage location for storing a beginning address of the undesired code, a second storage location for storing a beginning address of the replacement code, and a plurality of addressable storage locations for storing the replacement code wherein the last of said addressable storage locations stores a return instruction for readdressing said ROM;
• a program counter responsive to a central processing unit for addressing both said ROM and said electronically programmable memory;
• a comparator coupled to said first storage location and said program counter for detecting said beginning address of said undesired code;
• transfer logic responsive to said comparator and coupled to both said electronically programmable memory and said program counter for transferring said beginning address of said replacement code to said program counter;
• said program counter sequencing addresses through said plurality of addressable storage locations in said electronically programmable memory;
• and
• said central processing unit receiving code from said ROM and said electronically programmable memory, said central processing unit recognizing said return address for instructing said program counter to address said ROM after said replacement code has replaced said undesired code.

An apparatus for replacing an undesired code stored in a plurality of storage locations in read-only memory (ROM) with a replacement code, comprising:

• a first storage location for storing a beginning address of the undesired code;
• a second storage location for storing a beginning address of the replacement code;
• an electronically programmable memory having a plurality of addressable storage locations for storing the replacement code wherein the last of said addressable storage locations stores a jump-back instruction for readdressing said ROM;
• a program counter responsive to a central processing unit for addressing both said ROM and said electronically programmable memory;
• a comparator coupled to said first storage location and said program counter for detecting said beginning address of the undesired code;
• transfer logic responsive to said comparator and coupled to both said second storage location and said program counter for transferring said beginning address of the replacement code to said program counter;
• said program counter sequencing addresses through said plurality of addressable storage locations in said electronically programmable memory;
• and
• said central processing unit receiving code from said ROM and said electronically programmable memory, said central processing unit recognizing said jump-back instruction for instructing said program counter to address said ROM after said replacement code has replaced said undesired code.

A method for replacing an undesired code stored in a plurality of storage locations in a read-only memory (ROM) with a replacement code, comprising the steps of:

• storing a beginning address of the undesired code;
• storing a beginning address of the replacement code;
• storing the replacement code in a plurality of addressable storage locations in an electronically programmable memory wherein the last of said addressable storage locations stores a jump-back instruction for readdressing said ROM;
• addressing said ROM by a program counter responsive to a central processing unit;
• generating a transfer instruction when the address provided by said program counter compares to the stored address of the beginning location of the undesired code;
• transferring the stored beginning address of said replacement code to said program counter in response to said transfer instruction;
• sequencing addresses through said plurality of addressable storage locations in said electronically programmable memory by said program counter;
• and
• coupling said replacement code to a central processing unit for recognizing said jump-back instruction and readdressing said ROM after said replacement code has replaced said undesired code.