By Derrick Morris
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Extra info for An Introduction to System Programming — Based on the PDP11
Say . Its complete action Is PC <= the va lue of register R register R <= -[SP). That Is the specified register Is copied to PC. and the register Is then reset from the stack. It Is. therefore . necessary to advance the register R to the first Instruction after the 'Inllne' parameters If the facility to store parameters after the JSR Instruction Is exploited. This can sometimes be achieved by using auto-Increment at the time of accessing the parameters. Another Instruction which follows the modified double operand form Is the 'exclusive or ' Instruction (XOR) which Is an addition to the repertoire to logical Instructions .
In octal. to read In programs written In this better notation. Again practical considerations should moderate ambitions. and the step to the standard user languages such as Basic. Fortran or Pascal should not be taken In one stride. Usually the language used at this point Is a symbolic form of the machine Instructions which stili largely maintains a one-to-one correspondence between statements In the language and machine Instructions. At this stage of the bootstrap It Is often expedient to write the program to read In the symbolic programs (called an assembler) .
We shall consider paper tape . Characters punched on paper tape will have the codes given In Appendix 6. That Is. If the pattern of holes punched to represent say a '3' Is read as a number. Its value will be octal 063 . The conventions that a programmer Is to follow In preparing the paper tape must now be decided . It Is usual to have a definite character to mark the beginning of the Information on the paper tape rather than assume that the tape will be positioned In the reader exactly on the starting character .