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Compatibility Notes

This page lists current compatibility notes and scope limits based on the current codebase.

CPU Model Scope

  • The core exposes one execution profile through M68kCpu.
  • There is no public API to select CPU model variants (for example 68010/68020 mode switches).
  • Some later-family instructions exist (MOVEC, MOVES, RTD, BKPT), but model behavior is not fully parameterized.

Address Space and Memory Model

  • All memory accesses are against one flat memory buffer (cpu->memory, cpu->memory_size).
  • Addresses are masked to 24-bit (address & 0x00FFFFFF) before bounds checks.
  • Per-access host memory callbacks (m68k_set_read8_callback and related setters) can replace flat-buffer access for each width; see the API reference for details.

Callback Behavior Notes

  • fc_callback is emitted for memory reads/writes and instruction fetches.
  • M68K_FC_INT_ACK is defined, but the current interrupt acknowledge path does not emit FC callback events with this code.
  • pc_changed_callback is triggered when PC is changed through m68k_set_pc.
  • Direct PC writes (for example in m68k_reset and m68k_fetch) do not call pc_changed_callback.
  • reset_callback is tied to execution of the RESET instruction, not to m68k_reset().
  • illg_callback can be installed, but the current decode/exception path does not call it.

Control Registers and Exception Base

  • VBR, SFC, and DFC fields exist and are accessible through MOVEC.
  • Exception vector fetch currently uses vector * 4 from base address zero.
  • VBR is not currently applied as an exception vector base in m68k_exception.
  • SFC/DFC values are stored but not used to drive bus access behavior.

Context Save/Restore Format

  • m68k_get_context / m68k_set_context copy raw M68kCpu struct bytes.
  • The blob format should be treated as build-dependent (compiler/ABI/version sensitive), not a stable cross-version interchange format.
  • m68k_set_context preserves destination instance memory binding and installed callbacks.

Loader and Disassembler Notes

  • m68k_load_srec and m68k_load_bin return false only when file open fails.
  • m68k_load_srec reports malformed lines and continues parsing.
  • S-record checksum validity is not explicitly validated.
  • Loaders write directly into bound flat memory; they do not run emulated bus cycles, invoke host memory callbacks, or raise bus errors. When a record reaches an out-of-range address, the first out-of-range byte is reported to stderr, the rest of that record is skipped, and parsing continues with the next record.
  • S-record entry records (S7/S8/S9) set the program counter through m68k_set_pc.
  • m68k_disasm returns instruction bytes consumed; unsupported decode cases may still produce ??? output text.

JSON Compatibility Harness

  • The JSON compatibility runner (tests/test_json.c) has a relaxed default for exception-path state checks.
  • Tests skipped under the relaxed default are counted and reported per file.
  • Strict exception-path checking is available with ROCKET68_JSON_STRICT=1.
  • Per-test cycle count verification is available with ROCKET68_JSON_CYCLES=1.
  • This behavior is test-harness policy, not a runtime core API toggle.