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发表于 2004-8-29 18:34:00
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Flag (Processor Status) Instructions
Affect Flags: as noted
These instructions are implied mode, have a length of one byte and require two machine cycles.
MNEMONIC HEX
CLC (CLear Carry) $18
SEC (SEt Carry) $38
CLI (CLear Interrupt) $58
SEI (SEt Interrupt) $78
CLV (CLear oVerflow) $B8
CLD (CLear Decimal) $D8
SED (SEt Decimal) $F8
Notes:
The Interrupt flag is used to prevent (SEI) or enable (CLI) maskable interrupts (aka IRQ's). It does not signal the presence or absence of an interrupt condition. The 6502 will set this flag automatically in response to an interrupt and restore it to its prior status on completion of the interrupt service routine. If you want your interrupt service routine to permit other maskable interrupts, you must clear the I flag in your code.
The Decimal flag controls how the 6502 adds and subtracts. If set, arithmetic is carried out in packed binary coded decimal. This flag is unchanged by interrupts and is unknown on power-up. The implication is that a CLD should be included in boot or interrupt coding.
The Overflow flag is generally misunderstood and therefore under- utilised. Following addition or subtraction, the overflow will equal the EOR (exclusive or) of the Carry and Sign flags. This flag is not affected by increments, decrements, shifts and logical operations i.e. only ADC, SBC, CLV, PLP and RTI affect it. There is no op code to set the overflow but a BIT test on an RTS instruction will do the trick.
INC (INCrement memory)
Affects Flags: S Z
MODE SYNTAX HEX LEN TIM
Zero Page INC $44 $E6 2 5
Zero Page,X INC $44,X $F6 2 6
Absolute INC $4400 $EE 3 6
Absolute,X INC $4400,X $FE 3 7
JMP (JuMP)
Affects Flags: none
MODE SYNTAX HEX LEN TIM
Absolute JMP $5597 $4C 3 3
Indirect JMP ($5597) $6C 3 5
JMP transfers program execution to the following address (absolute) or to the location contained in the following address (indirect). Note that there is no carry associated with the indirect jump s
AN INDIRECT JUMP MUST NEVER USE A
VECTOR BEGINNING ON THE LAST BYTE
OF A PAGE
For example if address $3000 contains $40, $30FF contains $80, and $3100 contains $50, the result of JMP ($30FF) will be a transfer of control to $4080 rather than $5080 as you intended i.e. the 6502 took the low byte of the address from $30FF and the high byte from $3000.
JSR (Jump to SubRoutine)
Affects Flags: none
MODE SYNTAX HEX LEN TIM
Absolute JSR $5597 $20 3 6
JSR pushes the address-1 of the next operation on to the stack before transferring program control to the following address. Subroutines are normally terminated by a RTS op code.
LDA (LoaD Accumulator)
Affects Flags: S Z
MODE SYNTAX HEX LEN TIM
Immediate LDA #$44 $A9 2 2
Zero Page LDA $44 $A5 2 3
Zero Page,X LDA $44,X $B5 2 4
Absolute LDA $4400 $AD 3 4
Absolute,X LDA $4400,X $BD 3 4+
Absolute,Y LDA $4400,Y $B9 3 4+
Indirect,X LDA ($44,X) $A1 2 6
Indirect,Y LDA ($44),Y $B1 2 5+
+ add 1 cycle if page boundary crossed
LDX (LoaD X register)
Affects Flags: S Z
MODE SYNTAX HEX LEN TIM
Immediate LDX #$44 $A2 2 2
Zero Page LDX $44 $A6 2 3
Zero Page,Y LDX $44,Y $B6 2 4
Absolute LDX $4400 $AE 3 4
Absolute,Y LDX $4400,Y $BE 3 4+
+ add 1 cycle if page boundary crossed
LDY (LoaD Y register)
Affects Flags: S Z
MODE SYNTAX HEX LEN TIM
Immediate LDY #$44 $A0 2 2
Zero Page LDY $44 $A4 2 3
Zero Page,X LDY $44,X $B4 2 4
Absolute LDY $4400 $AC 3 4
Absolute,X LDY $4400,X $BC 3 4+
+ add 1 cycle if page boundary crossed
LSR (Logical Shift Right)
Affects Flags: S Z C
MODE SYNTAX HEX LEN TIM
Accumulator LSR A $4A 1 2
Zero Page LSR $44 $46 2 5
Zero Page,X LSR $44,X $56 2 6
Absolute LSR $4400 $4E 3 6
Absolute,X LSR $4400,X $5E 3 7
LSR shifts all bits right one position. 0 is shifted into bit 7 and the original bit 0 is shifted into the Carry.
Wrap-Around
Use caution with indexed zero page operations as they are subject to wrap-around. For example, if the X register holds $FF and you execute LDA $80,X you will not access $017F as you might expect; instead you access $7F i.e. $80-1. This characteristic can be used to advantage but make sure your code is well commented.
In cases where you are writing code that will be relocated you must consider wrap-around when assigning dummy values for addresses that will be adjusted. Both zero and the semi-standard $FFFF should be avoided for dummy labels. The use of zero or zero page values will result in assembled code with zero page opcodes when you wanted absolute codes. With $FFFF, the problem is in addresses+1 as you wrap around to page 0.
Program Counter
When the 6502 is ready for the next instruction it increments the program counter before fetching the instruction. Once it has the op code, it increments the program counter by the length of the operand, if any. This must be accounted for when calculating branches or when pushing bytes to create a false return address (i.e. jump table addresses are made up of addresses-1 when it is intended to use an RTS rather than a JMP).
The program counter is loaded least signifigant byte first. Therefore the most signifigant byte must be pushed first when creating a false return address.
When calculating branches a forward branch of 6 skips the following 6 bytes so, effectively the program counter points to the address that is 8 bytes beyond the address of the branch opcode; and a backward branch of $FA (256-6) goes to an address 7 bytes before the branch instruction.
Execution Times
Op code execution times are measured in machine cycles, one of which equals two clock cycles. Many instructions require one extra cycle for execution if a page boundary is crossed; these are indicated by a + following the time values shown.
NOP (No OPeration)
Affects Flags: none
MODE SYNTAX HEX LEN TIM
Implied NOP $EA 1 2
NOP is used to reserve space for future modifications or effectively REM out existing code.
ORA (bitwise OR with Accumulator)
Affects Flags: S Z
MODE SYNTAX HEX LEN TIM
Immediate ORA #$44 $09 2 2
Zero Page ORA $44 $05 2 2
Zero Page,X ORA $44,X $15 2 3
Absolute ORA $4400 $0D 3 4
Absolute,X ORA $4400,X $1D 3 4+
Absolute,Y ORA $4400,Y $19 3 4+
Indirect,X ORA ($44,X) $01 2 6
Indirect,Y ORA ($44),Y $11 2 5+
+ add 1 cycle if page boundary crossed
Register Instructions
Affect Flags: S Z
These instructions are implied mode, have a length of one byte and require two machine cycles.
MNEMONIC HEX
TAX (Transfer A to X) $AA
TXA (Transfer X to A) $8A
DEX (DEcrement X) $CA
INX (INcrement X) $E8
TAY (Transfer A to Y) $A8
TYA (Transfer Y to A) $98
DEY (DEcrement Y) $88
INY (INcrement Y) $C8
ROL (ROtate Left)
Affects Flags: S Z C
MODE SYNTAX HEX LEN TIM
Accumulator ROL A $2A 1 2
Zero Page ROL $44 $26 2 5
Zero Page,X ROL $44,X $36 2 6
Absolute ROL $4400 $2E 3 6
Absolute,X ROL $4400,X $3E 3 7
ROL shifts all bits left one position. The Carry is shifted into bit 0 and the original bit 7 is shifted into the Carry.
ROR (ROtate Right)
Affects Flags: S Z C
MODE SYNTAX HEX LEN TIM
Accumulator ROR A $6A 1 2
Zero Page ROR $44 $66 2 5
Zero Page,X ROR $44,X $76 2 6
Absolute ROR $4400 $6E 3 6
Absolute,X ROR $4400,X $7E 3 7
ROR shifts all bits right one position. The Carry is shifted into bit 7 and the original bit 0 is shifted into the Carry.
RTI (ReTurn from Interrupt)
Affects Flags: all
MODE SYNTAX HEX LEN TIM
Implied RTI $40 1 6
RTI retrieves the Processor Status Word (flags) and the Program Counter from the stack in that order (interrupts push the PC first and then the PSW).
Note that unlike RTS, the return address on the stack is the actual address rather than the address-1.
RTS (ReTurn from Subroutine)
Affects Flags: none
MODE SYNTAX HEX LEN TIM
Implied RTS $60 1 6
RTS pulls the top two bytes off the stack (low byte first) and transfers program control to that address+1. It is used, as expected, to exit a subroutine invoked via JSR which pushed the address-1.
RTS is frequently used to implement a jump table where addresses-1 are pushed onto the stack and accessed via RTS eg. to access the second of four routines:
LDX #1
JSR EXEC
JMP SOMEWHERE
LOBYTE
.BYTE <ROUTINE0-1,<ROUTINE1-1
.BYTE <ROUTINE2-1,<ROUTINE3-1
HIBYTE
.BYTE >ROUTINE0-1,>ROUTINE1-1
.BYTE >ROUTINE2-1,>ROUTINE3-1
EXEC
LDA HIBYTE,X
PHA
LDA LOBYTE,X
PHA
RTS
SBC (SuBtract with Carry)
Affects Flags: S V Z C
MODE SYNTAX HEX LEN TIM
Immediate SBC #$44 $E9 2 2
Zero Page SBC $44 $E5 2 3
Zero Page,X SBC $44,X $F5 2 4
Absolute SBC $4400 $ED 3 4
Absolute,X SBC $4400,X $FD 3 4+
Absolute,Y SBC $4400,Y $F9 3 4+
Indirect,X SBC ($44,X) $E1 2 6
Indirect,Y SBC ($44),Y $F1 2 5+
+ add 1 cycle if page boundary crossed
SBC results are dependant on the setting of the decimal flag. In decimal mode, subtraction is carried out on the assumption that the values involved are packed BCD (Binary Coded Decimal).
There is no way to subtract without the carry which works as an inverse borrow. i.e, to subtract you set the carry before the operation. If the carry is cleared by the operation, it indicates a borrow occurred.
STA (STore Accumulator)
Affects Flags: none
MODE SYNTAX HEX LEN TIM
Zero Page STA $44 $85 2 3
Zero Page,X STA $44,X $95 2 4
Absolute STA $4400 $8D 3 4
Absolute,X STA $4400,X $9D 3 5
Absolute,Y STA $4400,Y $99 3 5
Indirect,X STA ($44,X) $81 2 6
Indirect,Y STA ($44),Y $91 2 6
Return To Index
Stack Instructions
These instructions are implied mode, have a length of one byte and require machine cycles as indicated. The "PuLl" operations are known as "POP" on most other microprocessors. With the 6502, the stack is always on page one ($100-$1FF) and works top down.
MNEMONIC HEX TIM
TXS (Transfer X to Stack ptr) $9A 2
TSX (Transfer Stack ptr to X) $BA 2
PHA (PusH Accumulator) $48 3
PLA (PuLl Accumulator) $68 4
PHP (PusH Processor status) $08 3
PLP (PuLl Processor status) $28 4
STX (STore X register)
Affects Flags: none
MODE SYNTAX HEX LEN TIM
Zero Page STX $44 $86 2 3
Zero Page,Y STX $44,Y $96 2 4
Absolute STX $4400 $8E 3 4
STY (STore Y register)
Affects Flags: none
MODE SYNTAX HEX LEN TIM
Zero Page STY $44 $84 2 3
Zero Page,X STY $44,X $94 2 4
Absolute STY $4400 $8C 3 4
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发表于 2004-8-29 18:37:00
发表于 2004-10-25 12:31:00
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