Fix a bug in dcpu_skip, add a test program. #15
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added 5 commits
April 17, 2012 22:33
misses the pc increment for the b operand in a basic instruction. The test shows that a 3-word instruction is not properly skipped. The 3rd word of the instruction (SET [0x1000], [0xEEE0]), when mistakenly interpreted as the 1st word of the next instruction, causes an illegal opcode bailout in the monitor.
…l non-basic opcodes to use as an emulator success code.
… for non-basic opcodes) could overflow skiptable, when used as an index. Masking the operand to force it within the table size has its own problem, as it causes aliasing from some constant operands to operands which "advance pc". Example: c00d 009c: IFN A, 16 fc03 009d: SUB A, 31 The constant 31 in the second instruction is 'b' operand value 0x3F. If masked with 31 (0x1F) to stay within the table, it becomes 0x1F, which has value 1 within skiptable - that's incorrect, the constant does not require another instruction word. The solution in this commit tries to perturb the existing code as little as possible; I just bump skiptable up to 64-locations, so there's no need to mask the operand before using it as an index. The other change is to update the mask on the basic-opcode 'a' operand so that literals are handled correcty for that operand as well. I've included a test case, "testskip3.s", which fails before the fix and passes after the fix. All 32 literal operand values for the 'b' operand are tested.
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I've found and fixed a big in dcpu_skip, have a look and see if you think it's worth merging with your main branch. I've included a test which fails with the bug, and passes with my fix.