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105 lines
3.7 KiB
C
105 lines
3.7 KiB
C
/*
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* Sim65 cpu-mode switching example.
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*
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* Description
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* -----------
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*
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* We can inspect and manipulate the CPU model that sim65 emulates at runtime.
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*
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* Sim65 always runs in one of three modes:
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*
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* - 6502 mode: the 151 documented opcodes are supported; if the processor encounters
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* one of the 105 undocumented opcodes, the simulator ends with an
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* 'illegal opcode' message.
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* - 65C02 mode: the 105 undocumented opcodes now have well-defined behavior. Some
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* do useful things, while all others are now defined as NOPs.
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* - 6502X mode: the 105 undocumented opcodes don't have documented behavior, but
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* they /do/ have behavior on a real 6502. This behavior has been
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* figured out, and is deterministic (with minor exceptions).
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* In this mode, sim65 mimics the behavior of a real 6502 when
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* it encounters an undocumented opcode, rather than terminating.
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*
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* In the example below, we first switch to 6502X mode and execute a small
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* assembly code fragment, then repeat this in 65C02 mode.
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*
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* The code fragment is designed to distinguish between a 6502 and a 65C02
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* processor based on the behavior of the ADC function in decimal mode.
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*
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* Important Note:
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*
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* When running in a program compiled for the "sim6502" target, it is safe to switch to
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* 65C02 or 6502X mode, since the runtime library will only use plain 6502 opcodes, and
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* those work the same in 65C02 and 6502X mode.
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*
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* However, when running in a program compiled for the "sim65c02" target, it is NOT safe
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* to switch to 6502 or 6502X mode, since many routines in the runtime library use
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* 65C02-specific opcodes, and these will not work as expected when the CPU is switched
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* to 6502 or 6502X mode. When such an instruction is encountered, the program will
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* exhibit undefined behavior.
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*
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* For this reason, this program will only work when compiled for the "sim6502" target.
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*
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* Running the example
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* -------------------
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*
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* cl65 -t sim6502 -O cpumode_example.c -o cpumode_example.prg
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* sim65 cpumode_example.prg
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*
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*/
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#include <stdio.h>
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#include <stdbool.h>
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#include <sim65.h>
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static bool __fastcall__ is_65c02(void)
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{
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/* This assembly routine loads 0 into AX on a 6502 (also on a 6502 on which decimal
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* mode is not implemented), and 1 on a 65C02.
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*
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* Note: this implementation triggers a "control reaches end of non-void function"
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* warning that can be safely ignored. While no return statement is present, the
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* return value is correctly loaded into AX by the assembly code.
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*/
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__asm__("sed");
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__asm__("ldx #0");
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__asm__("txa");
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__asm__("sbc #155");
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__asm__("asl");
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__asm__("rol");
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__asm__("and #1");
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__asm__("cld");
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}
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int main(void)
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{
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printf("CPU mode at startup ....... : %u\n", GET_CPU_MODE());
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printf("Is 65C02? ................. : %s\n", is_65c02() ? "YES" : "NO");
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printf("\n");
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printf("Switching to 6502 mode ....\n");
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SET_CPU_MODE(SIM65_CPU_MODE_6502);
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printf("Current CPU mode .......... : %u\n", GET_CPU_MODE());
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printf("Is 65C02? ................. : %s\n", is_65c02() ? "YES" : "NO");
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printf("\n");
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printf("Switching to 65C02 mode ...\n");
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SET_CPU_MODE(SIM65_CPU_MODE_65C02);
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printf("Current CPU mode .......... : %u\n", GET_CPU_MODE());
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printf("Is 65C02? ................. : %s\n", is_65c02() ? "YES" : "NO");
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printf("\n");
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printf("Switching to 6502X mode ...\n");
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SET_CPU_MODE(SIM65_CPU_MODE_6502X);
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printf("Current CPU mode .......... : %u\n", GET_CPU_MODE());
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printf("Is 65C02? ................. : %s\n", is_65c02() ? "YES" : "NO");
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printf("\n");
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printf("Bye!\n");
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return 0;
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}
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