An Introduction to System Programming — Based on the PDP11 by Derrick Morris

By Derrick Morris

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It Is called Jump to SUbroutine and has the ' mnemonic JSR. Two actions result from the execution of this Instruction . First. after the operand address has been evaluated . the value of the program counter (PC) Is stacked. It wil l at this time be the address of the next Instruction and the stacked copy Is called a 'link' . Next the program counter Is reset to the address of the operand . Thus If the Instruction 004737 002000 was In store lines 1000 and 1002, Its execution would ba aqulvalenl 10 the two Instructions 012746 001004 012707 002000 -[SP)<=1004 PC<=2000 It should now be obvious that the Instruction required at the end of the subroutine must perform the action PC<=[SP)+ .

Some discussion is necessary for the other two groups The Structure 01 the PDP11 37 which perform multiple tests. The group BGE . BlT. BGT and BlE are used for quite straightforward comparisons but the means by which they achieve the required effects are possibly obscure . Consider BGE which branches If the last result was greater than or equal to zero . This might be used after comparing two numbers . X and Y say . by means of the compare Instruction CMP X Y. which will subtract Y from X. As we would expect .

It Is called Jump to SUbroutine and has the ' mnemonic JSR. Two actions result from the execution of this Instruction . First. after the operand address has been evaluated . the value of the program counter (PC) Is stacked. It wil l at this time be the address of the next Instruction and the stacked copy Is called a 'link' . Next the program counter Is reset to the address of the operand . Thus If the Instruction 004737 002000 was In store lines 1000 and 1002, Its execution would ba aqulvalenl 10 the two Instructions 012746 001004 012707 002000 -[SP)<=1004 PC<=2000 It should now be obvious that the Instruction required at the end of the subroutine must perform the action PC<=[SP)+ .

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