95 lines
3.7 KiB
Go
95 lines
3.7 KiB
Go
// Package vm emulates a MOS Technology 6502 Microprocessor which is an 8-bit microprocessor
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// that was designed by a small team led by Chuck Peddle for MOS Technology. The design team
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// had formerly worked at Motorola on the Motorola 6800 project; the 6502 is essentially
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// simplified, less expensive and faster version of that design.
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//
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// A .... Accumulator OPC A operand is AC (implied single byte instruction)
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// abs .... absolute OPC $LLHH operand is address $HHLL *
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// abs,X .... absolute, X-indexed OPC $LLHH,X operand is address; effective address is address incremented by X with carry **
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// abs,Y .... absolute, Y-indexed OPC $LLHH,Y operand is address; effective address is address incremented by Y with carry **
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// # .... immediate OPC #$BB operand is byte BB
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// impl .... implied OPC operand implied
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// ind .... indirect OPC ($LLHH) operand is address; effective address is contents of word at address: C.w($HHLL)
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// X,ind .... X-indexed, indirect OPC ($LL,X) operand is zeropage address; effective address is word in (LL + X, LL + X + 1), inc. without carry: C.w($00LL + X)
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// ind,Y .... indirect, Y-indexed OPC ($LL),Y operand is zeropage address; effective address is word in (LL, LL + 1) incremented by Y with carry: C.w($00LL) + Y
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// rel .... relative OPC $BB branch target is PC + signed offset BB ***
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// zpg .... zeropage OPC $LL operand is zeropage address (hi-byte is zero, address = $00LL)
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// zpg,X .... zeropage, X-indexed OPC $LL,X operand is zeropage address; effective address is address incremented by X without carry **
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// zpg,Y .... zeropage, Y-indexed OPC $LL,Y operand is zeropage address; effective address is address incremented by Y without carry **
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//
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// 16-bit address words are little endian, lo(w)-byte first, followed by the hi(gh)-byte.
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//
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// Processor Stack:
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// LIFO, top down, 8 bit range, 0x0100 - 0x01FF
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//
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// 6502 instructions have the general form AAABBBCC, where AAA and CC define the opcode, and BBB defines the addressing mode
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package vm
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// addrMode is a type alias for a string, used below for defining addressing modes
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type addrMode int
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const (
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accumulator addrMode = iota
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absolute
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absoluteXIndexed
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absoluteYIndexed
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immediate
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implied
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indirect
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indirectXIndexed
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indirectYIndexed
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relative
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zeroPage
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zeroPageXIndexed
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zeroPageYIndexed
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)
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// Available cpu flags written as binary integer literals
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// https://wiki.nesdev.com/w/index.php/Status_flags
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// 7 bit 0
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// - - - - - - - -
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// N V s s D I Z C
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// | | | | | | | |
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// | | | | | | | +- Carry
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// | | | | | | +--- Zero
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// | | | | | + ---- Interrupt Disable
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// | | | | + ------ Decimal
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// | | + +--------- No CPU effect, see: the B flag
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// | + ------------ Overflow
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// +--------------- Negative
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const (
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flagDefault byte = 0B_00110000
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flagNegative byte = 0B_10000000
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flagOverflow byte = 0B_01000000
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flagBreak byte = 0B_00010000
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flagDecimalMode byte = 0B_00001000
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flagDisableInterrupts byte = 0B_00000100
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flagZero byte = 0B_00000010
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flagCarry byte = 0B_00000001
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)
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// StackBottom represents the start of the stack
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const StackBottom uint16 = 0x0100 // 256
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// Mos6502 represents the cpu's registers
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type Mos6502 struct {
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sp byte // register - stack pointer
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pc uint16 // register - program counter
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a byte // register - accumulator
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x byte // register - x index
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y byte // register - y index
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ps byte // register - processor status
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}
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// newCPU initializes and returns a new Mos6502 CPU
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func newCPU() *Mos6502 {
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return &Mos6502{
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sp: 0xFF,
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pc: 0,
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a: 0,
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x: 0,
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y: 0,
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ps: flagDefault,
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}
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}
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