By Richard Mansfield
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Extra resources for Second Book of Machine Language: Personal Computer Machine Language Programming for the Commodre 64, VIC-20, Atari, Apple, and PET CBM Computers
A single byte is all that can be used as a branch argument. To make it a correct branch argument, we've got to subtract 2 from it. This, you recall, is because all branches are calculated from the address of the mnemonic which comes just after the branch instruction. Counting starts from the B instruction 's address, plus two. Subtracting two will fix this up for branches in either direction. Further Evaluation We've seen how LADS calculates the branch addresses. At this point in the source code, we come upon a continuation of evaluations of other addressing modes.
Then we check the low-byte result of the subtraction (which was pushed on the stack temporarily in line 2500) to see if it's a correct value . The PLA (2580) will set the N flag in the Status Register if the number is greater than 127. We want it to be, since this is a backward branch. If this flag is not set, we BPL to the error message . Otherwise, we jump to the concluding routine, setting up a correct branch. The FOR routine handles forward branches in a similar way, going to the error routine if the high byte is not zero (2610) or if the low byte has the seventh bit set (proving it's greater than 127, an incorrect forward branch).
This SMORE routine (370) will be used again when we've completed the first pass of the assembly process. The first pass goes through the entire source file, storing all the names of the labels and their numeric values into an array. When we finish making this collection of labels, our label array, we've got to make a second pass, filling in the opcodes and replacing those labels with numbers . It's here, at SMORE, that we jump to start the second pass. A zero is given to ENDFLAG to keep the assembler running.
Second Book of Machine Language: Personal Computer Machine Language Programming for the Commodre 64, VIC-20, Atari, Apple, and PET CBM Computers by Richard Mansfield