76 lines
3.2 KiB
C
76 lines
3.2 KiB
C
// cpu.h
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// SplitBit CPU Emulator Core
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// Written by Anachronaut
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// 10/16/2024
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#ifndef CPU_H
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#define CPU_H
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#include <stdint.h>
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// How many Data Pointers the CPU has. The instructions that name one take a full
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// byte to do it, so the encoding would allow up to 256. The limit here is the size
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// of the register file and the cost of saving pointers across a CALL, not the
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// instruction format. Must be a power of two, so that the selector can be masked
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// down to a valid pointer.
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#define DATA_POINTERS 4
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// How many Data Pointers survive a CALL. The low numbered pointers are saved and
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// restored around a subroutine; the rest are left alone, so a subroutine can use
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// one to hand a pointer back to its caller the way Q hands back a byte. This is
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// deliberately independent of DATA_POINTERS: adding more pointers should not make
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// every CALL more expensive.
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#define PRESERVED_DATA_POINTERS 3
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#if PRESERVED_DATA_POINTERS > DATA_POINTERS
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#error "Cannot preserve more Data Pointers than the CPU has."
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#endif
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// The bits of the Status register that mean something.
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#define STATUS_CARRY 0x01 // An arithmetic result carried out of, or borrowed into, a byte.
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#define STATUS_FAULT 0x02 // The CPU met a byte it could not decode, and stopped.
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#define STATUS_INTERRUPT 0x04 // Hardware interrupts are enabled. Nothing reads this yet.
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#define STATUS_HALT 0x80 // Execution has stopped, either from HALT or from a fault.
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// What an interrupt puts on the Stack: the resume address, every Data Pointer, and
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// every register the CPU has. The CALL frame leaves Q and DP3 alone, but that is a
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// convention between a caller and the subroutine it called. An interrupt arrives in
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// code that never agreed to give anything up, so it saves the lot.
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#define INTERRUPT_FRAME_BYTES (2 + DATA_POINTERS * 2 + 4)
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// Why the CPU stopped, when the Fault Flag is set. This is not something a program can
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// read, and it is deliberately not a register: when a handler is installed, the vector
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// it arrived through already says what happened, which is why the ISA has no fault
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// cause. This exists for the case where nothing is installed and the machine is dead,
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// so that whatever examines the wreckage can say something better than "it stopped".
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typedef enum {
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FAULT_NONE = 0,
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FAULT_BAD_OPCODE, // A byte that does not decode to an instruction.
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FAULT_NO_HANDLER, // Dispatched through a software vector with nothing in it.
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FAULT_NO_DEVICE_HANDLER // A device interrupted, and its vector was empty.
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} FaultCause;
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// The struct containing the CPU registers.
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typedef struct {
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uint8_t A;
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uint8_t B;
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uint8_t Q;
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uint8_t Status;
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uint16_t ProgramCounter;
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uint16_t DataPointer[DATA_POINTERS];
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uint16_t StackPointer;
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uint8_t *Program;
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uint8_t *Data;
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// Set alongside the Fault Flag, and read only by whatever reports the stop.
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uint8_t Fault; // A FaultCause.
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uint8_t FaultVector; // Which vector was empty, when Fault is FAULT_NO_HANDLER.
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} CPURegisters;
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uint8_t executeOperation(uint8_t instruction, CPURegisters *cpu);
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void initializeCPU(CPURegisters *cpu, uint8_t *programMemory, uint8_t *dataMemory);
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void stepCPU(CPURegisters *cpu);
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#endif // CPU_H
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