"return" statement by silently adding "return 0" instead of emitting a
warning. This is the desired behavior for the "main" function in C99 and
above, but the compiler applied it to all functions.
This PR is the first of two PRs that replaces earlier PRs #2589 and #2590.
Due to a git branching mishap it was decided to re-partition the new
functionality in two sequential PRs that offer self-contained, new
functionality to sim65.
The functionality in this first PR extends the sim65 simulator in the following ways:
(1) It provides tracing functionality, i.e., the possibility of printing one line of simulator state information per instruction executed.
(2) It provides a memory mapped "sim65 control" peripheral that allows control of (a) the tracing functionality, and (b) the cpu mode.
(3) It provides command-line options to sim65 to enable the tracing, and to override the CPU mode as specified in the program file header.
More detailed information and some discussion can be found in the discussions with the (now retracted) PRs #2589 and #2590.
This PR provides the technical infrastructure inside the sim65 simulator program itself. Once this PR is accepted, a follow-up PR will be posted that adds C and assembly-language support for the new tracing and peripheral features so they can be easily accessed from the CC65 compiler and the CA65 assembler; some examples; and the documentation for these features. The lack of the latter, in this pull request, will be addressed then.
This PR fixes all discrepancies of sim65 instruction timings, for both the 6502 and the 65C02 processors.
The timings as implemented in this PR have been verified against actual hardware (Atari 800 XL for 6502; and WDC 65C02 for 65C02).
These timings can also be verified against the 65x02 test suite. However, in this case, a single discrepancy arises; the 65x02 testsuite suggests that the 65C02 opcode 0x5c should take 4 clocks. However, tests on a hardware 65C02 have conclusively shown that this instruction takes 8 clock cycles. The 8 clock cycles duration for the 65C02 0xfc opcode is also confirmed by other sources, e.g. Section 9 of http://www.6502.org/tutorials/65c02opcodes.html.
This test makes sim65 correct both in terms of functionality (all opcodes now do what they do on hardware) and in terms of timing (all instructions take as long as they would on real hardware).
The one discrepancy that remains, is that on a real 6502/65C02, some instructions issue R or W cycles on the bus while the instruction processing is being done. Those spurious bus cycles are not replicated in sim65. Sim65 is thus an instruction-level simulator, rather than a bus-cycle level simulator. In other words, while the clock cycle counts for each instruction are now correct, not all clock cycles are individually simulated.
