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diff --git a/doc/6502.org_ Tutorials and Aids.html b/doc/6502.org_ Tutorials and Aids.html new file mode 100644 index 0000000..f9a597f --- /dev/null +++ b/doc/6502.org_ Tutorials and Aids.html @@ -0,0 +1,635 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<!-- saved from url=(0046)http://www.6502.org/tutorials/6502opcodes.html --> +<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><title>6502.org: Tutorials and Aids</title> +<meta name="description" content="Learn how to do all kinds of things with the 6502 microprocessor."> + +</head> +<body> +<a href="http://www.6502.org/">[Return to Main Page]</a> +<font size="+1">NMOS 6502 Opcodes</font> <font size="-1">by John Pickens, +Updated by Bruce Clark and by Ed Spittles</font> +<br><a href="http://www.6502.org/tutorials/tutorials.htm">[Up to Tutorials and Aids]</a> +<br> +<hr> +<h2>INDEX</h2> +<table border="2" cellpadding="2" width="90%" style="font-family:'Lucida Console', monospace"> + <tbody> + <tr> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#BRA">Branches</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#DFLAG">Decimal Mode</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#IFLAG">Interrupt Flag</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#VFLAG">Overflow Flag</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#PC">Program + Counter</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#STACK">Stack</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#TIMES">Times</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#WRAP">Wrap-around</a></td></tr></tbody></table> +<p> +</p><table border="2" cellpadding="2" width="90%"> + <tbody> + <tr> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#ADC">ADC</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#AND">AND</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#ASL">ASL</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#BCC">BCC</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#BCS">BCS</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#BEQ">BEQ</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#BIT">BIT</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#BMI">BMI</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#BNE">BNE</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#BPL">BPL</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#BRK">BRK</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#BVC">BVC</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#BVS">BVS</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#CLC">CLC</a></td></tr> + <tr> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#CLD">CLD</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#CLI">CLI</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#CLV">CLV</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#CMP">CMP</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#CPX">CPX</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#CPY">CPY</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#DEC">DEC</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#DEX">DEX</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#DEY">DEY</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#EOR">EOR</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#INC">INC</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#INX">INX</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#INY">INY</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#JMP">JMP</a></td></tr> + <tr> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#JSR">JSR</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#LDA">LDA</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#LDX">LDX</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#LDY">LDY</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#LSR">LSR</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#NOP">NOP</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#ORA">ORA</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#PHA">PHA</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#PHP">PHP</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#PLA">PLA</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#PLP">PLP</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#ROL">ROL</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#ROR">ROR</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#RTI">RTI</a></td></tr> + <tr> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#RTS">RTS</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#SBC">SBC</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#SEC">SEC</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#SED">SED</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#SEI">SEI</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#STA">STA</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#STX">STX</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#STY">STY</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#TAX">TAX</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#TAY">TAY</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#TSX">TSX</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#TXA">TXA</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#TXS">TXS</a></td> + <td align="center"><a href="http://www.6502.org/tutorials/6502opcodes.html#TYA">TYA</a></td></tr></tbody></table> +<p><a name="ADC"> </a> +</p><h2>ADC (ADd with Carry)</h2> +<p>Affects Flags: N V Z C </p><pre>MODE SYNTAX HEX LEN TIM +Immediate ADC #$44 $69 2 2 +Zero Page ADC $44 $65 2 3 +Zero Page,X ADC $44,X $75 2 4 +Absolute ADC $4400 $6D 3 4 +Absolute,X ADC $4400,X $7D 3 4+ +Absolute,Y ADC $4400,Y $79 3 4+ +Indirect,X ADC ($44,X) $61 2 6 +Indirect,Y ADC ($44),Y $71 2 5+ + ++ add 1 cycle if page boundary crossed + +</pre>ADC results are dependant on the setting of the <a href="http://www.6502.org/tutorials/6502opcodes.html#DFLAG">decimal flag</a>. In decimal +mode, addition is carried out on the assumption that the values involved are +packed BCD (Binary Coded Decimal). +<p>There is no way to add without carry. + +</p><p><a name="AND"> </a> +</p><h2>AND (bitwise AND with accumulator) </h2> +<p>Affects Flags: N Z </p><pre>MODE SYNTAX HEX LEN TIM +Immediate AND #$44 $29 2 2 +Zero Page AND $44 $25 2 3 +Zero Page,X AND $44,X $35 2 4 +Absolute AND $4400 $2D 3 4 +Absolute,X AND $4400,X $3D 3 4+ +Absolute,Y AND $4400,Y $39 3 4+ +Indirect,X AND ($44,X) $21 2 6 +Indirect,Y AND ($44),Y $31 2 5+ + ++ add 1 cycle if page boundary crossed +</pre> + +<p><a name="ASL"> </a> +</p><h2>ASL (Arithmetic Shift Left) </h2> +<p>Affects Flags: N Z C </p><pre>MODE SYNTAX HEX LEN TIM +Accumulator ASL A $0A 1 2 +Zero Page ASL $44 $06 2 5 +Zero Page,X ASL $44,X $16 2 6 +Absolute ASL $4400 $0E 3 6 +Absolute,X ASL $4400,X $1E 3 7 + +</pre>ASL shifts all bits left one position. 0 is shifted into bit 0 and the +original bit 7 is shifted into the Carry. +<p> +</p><p><a name="BIT"> </a> +</p><h2>BIT (test BITs) </h2> +<p>Affects Flags: N V Z </p><pre>MODE SYNTAX HEX LEN TIM +Zero Page BIT $44 $24 2 3 +Absolute BIT $4400 $2C 3 4 + +</pre>BIT sets the Z flag as though the value in the address tested were ANDed +with the accumulator. The N and V flags are set to match bits 7 and 6 +respectively in the value stored at the tested address. +<p>BIT is often used to skip one or two following bytes as in: +</p><p></p><pre>CLOSE1 LDX #$10 If entered here, we + .BYTE $2C effectively perform +CLOSE2 LDX #$20 a BIT test on $20A2, + .BYTE $2C another one on $30A2, +CLOSE3 LDX #$30 and end up with the X +CLOSEX LDA #12 register still at $10 + STA ICCOM,X upon arrival here. + +</pre> +Beware: a BIT instruction used in this way as a NOP does have effects: the flags +may be modified, and the read of the absolute address, if it happens to access an +I/O device, may cause an unwanted action. +<p><a name="BCC"> </a> <a name="BCS"> </a> <a name="BEQ"> </a> <a name="BNE"> </a> <a name="BMI"> </a> <a name="BPL"> </a> <a name="BVC"> </a> <a name="BVS"> </a> <a name="BRA"> </a> +</p><h2>Branch Instructions</h2> +<p>Affect Flags: none +</p><p>All branches are relative mode and have a length of two bytes. Syntax is "Bxx +Displacement" or (better) "Bxx Label". See the notes on the <a href="http://www.6502.org/tutorials/6502opcodes.html#PC">Program Counter</a> for more on +displacements. +</p><p>Branches are dependant on the status of the flag bits when the op code is +encountered. A branch not taken requires two machine cycles. Add one if the +branch is taken and add one more if the branch crosses a page boundary. </p><pre>MNEMONIC HEX +BPL (Branch on PLus) $10 +BMI (Branch on MInus) $30 +BVC (Branch on oVerflow Clear) $50 +BVS (Branch on oVerflow Set) $70 +BCC (Branch on Carry Clear) $90 +BCS (Branch on Carry Set) $B0 +BNE (Branch on Not Equal) $D0 +BEQ (Branch on EQual) $F0 + +</pre>There is no BRA (BRanch Always) instruction but it can be easily emulated +by branching on the basis of a known condition. One of the best flags to use for +this purpose is the <a href="http://www.6502.org/tutorials/6502opcodes.html#VFLAG">oVerflow</a> which is unchanged +by all but addition and subtraction operations. +<p> +A page boundary crossing occurs when the branch destination is on a different +page than the instruction AFTER the branch instruction. For example: +</p><pre> SEC + BCS LABEL + NOP +</pre> +A page boundary crossing occurs (i.e. the BCS takes 4 cycles) when (the +address of) LABEL and the NOP are on different pages. This means that +<pre> CLV + BVC LABEL + LABEL NOP +</pre> +the BVC instruction will take 3 cycles no matter what address it is located +at. +<p> +</p><p><a name="BRK"> </a> +</p><h2>BRK (BReaK) </h2> +<p>Affects Flags: B </p><pre>MODE SYNTAX HEX LEN TIM +Implied BRK $00 1 7 + +</pre>BRK causes a non-maskable interrupt and increments the program counter by +one. Therefore an <a href="http://www.6502.org/tutorials/6502opcodes.html#RTI">RTI</a> will +go to the address of the BRK +2 so that BRK may be used to replace a +two-byte instruction for debugging and the subsequent RTI will be correct. +<p> +</p><p><a name="CMP"> </a> +</p><h2>CMP (CoMPare accumulator) </h2> +<p>Affects Flags: N Z C </p><pre>MODE SYNTAX HEX LEN TIM +Immediate CMP #$44 $C9 2 2 +Zero Page CMP $44 $C5 2 3 +Zero Page,X CMP $44,X $D5 2 4 +Absolute CMP $4400 $CD 3 4 +Absolute,X CMP $4400,X $DD 3 4+ +Absolute,Y CMP $4400,Y $D9 3 4+ +Indirect,X CMP ($44,X) $C1 2 6 +Indirect,Y CMP ($44),Y $D1 2 5+ + ++ add 1 cycle if page boundary crossed + +</pre>Compare sets flags as if a subtraction had been carried out. If the value +in the accumulator is equal or greater than the compared value, the Carry will +be set. The equal (Z) and negative (N) flags will be set based on equality or lack +thereof and the sign (i.e. A>=$80) of the accumulator. +<p> +</p><p><a name="CPX"> </a> +</p><h2>CPX (ComPare X register) </h2> +<p>Affects Flags: N Z C </p><pre>MODE SYNTAX HEX LEN TIM +Immediate CPX #$44 $E0 2 2 +Zero Page CPX $44 $E4 2 3 +Absolute CPX $4400 $EC 3 4 + +</pre>Operation and flag results are identical to equivalent mode accumulator <a href="http://www.6502.org/tutorials/6502opcodes.html#CMP">CMP</a> ops. +<p> +</p><p><a name="CPY"> </a> +</p><h2>CPY (ComPare Y register) </h2> +<p>Affects Flags: N Z C </p><pre>MODE SYNTAX HEX LEN TIM +Immediate CPY #$44 $C0 2 2 +Zero Page CPY $44 $C4 2 3 +Absolute CPY $4400 $CC 3 4 + +</pre>Operation and flag results are identical to equivalent mode accumulator <a href="http://www.6502.org/tutorials/6502opcodes.html#CMP">CMP</a> ops. +<p> +</p><p><a name="DEC"> </a> +</p><h2>DEC (DECrement memory) </h2> +<p>Affects Flags: N Z </p><pre>MODE SYNTAX HEX LEN TIM +Zero Page DEC $44 $C6 2 5 +Zero Page,X DEC $44,X $D6 2 6 +Absolute DEC $4400 $CE 3 6 +Absolute,X DEC $4400,X $DE 3 7 + +</pre> +<p><a name="EOR"> </a> +</p><h2>EOR (bitwise Exclusive OR) </h2> +<p>Affects Flags: N Z </p><pre>MODE SYNTAX HEX LEN TIM +Immediate EOR #$44 $49 2 2 +Zero Page EOR $44 $45 2 3 +Zero Page,X EOR $44,X $55 2 4 +Absolute EOR $4400 $4D 3 4 +Absolute,X EOR $4400,X $5D 3 4+ +Absolute,Y EOR $4400,Y $59 3 4+ +Indirect,X EOR ($44,X) $41 2 6 +Indirect,Y EOR ($44),Y $51 2 5+ + ++ add 1 cycle if page boundary crossed +</pre> +<p><a name="CLC"> </a> <a name="SEC"> </a> <a name="CLD"> </a> <a name="SED"> </a> <a name="CLI"> </a> <a name="SEI"> </a> <a name="CLV"> </a> +</p><h2>Flag (Processor Status) Instructions</h2> +<p>Affect Flags: as noted +</p><p>These instructions are implied mode, have a length of one byte and require +two machine cycles. </p><pre>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 + +</pre>Notes: +<p><a name="IFLAG"> </a> 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. +</p><p><a name="DFLAG"> </a> 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. +</p><p><a name="VFLAG"> </a> The Overflow flag is generally misunderstood and +therefore under-utilised. After an ADC or SBC instruction, the overflow flag +will be set if the twos complement result is less than -128 or greater than ++127, and it will cleared otherwise. In twos complement, $80 through $FF +represents -128 through -1, and $00 through $7F represents 0 through +127. +Thus, after: +</p><pre> CLC + LDA #$7F ; +127 + ADC #$01 ; + +1 +</pre> +the overflow flag is 1 (+127 + +1 = +128), and after: +<pre> CLC + LDA #$81 ; -127 + ADC #$FF ; + -1 +</pre> +the overflow flag is 0 (-127 + -1 = -128). The overflow flag is not +affected by increments, decrements, shifts and logical operations i.e. only +ADC, BIT, CLV, PLP, RTI and SBC affect it. There is no op code to set the +overflow but a BIT test on an RTS instruction will do the trick. +<p> +</p><p> +</p><p><a name="INC"> </a> +</p><h2>INC (INCrement memory) </h2> +<p>Affects Flags: N Z </p><pre>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 +</pre> +<p> +</p><p> +</p><p><a name="JMP"> </a> +</p><h2>JMP (JuMP) </h2> +<p>Affects Flags: none </p><pre>MODE SYNTAX HEX LEN TIM +Absolute JMP $5597 $4C 3 3 +Indirect JMP ($5597) $6C 3 5 + +</pre>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 so: <pre><strong>AN INDIRECT JUMP MUST NEVER USE A +VECTOR BEGINNING ON THE LAST BYTE +OF A PAGE</strong> +</pre>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. +<p> +</p><p> +</p><p><a name="JSR"> </a> +</p><h2>JSR (Jump to SubRoutine) </h2> +<p>Affects Flags: none </p><pre>MODE SYNTAX HEX LEN TIM +Absolute JSR $5597 $20 3 6 + +</pre>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 <a href="http://www.6502.org/tutorials/6502opcodes.html#RTS">RTS</a> op +code. +<p> +</p><p> +</p><p><a name="LDA"> </a> +</p><h2>LDA (LoaD Accumulator) </h2> +<p>Affects Flags: N Z </p><pre>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 +</pre> +<p> +</p><p> +</p><p><a name="LDX"> </a> +</p><h2>LDX (LoaD X register) </h2> +<p>Affects Flags: N Z </p><pre>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 +</pre> +<p> +</p><p> +</p><p><a name="LDY"> </a> +</p><h2>LDY (LoaD Y register) </h2> +<p>Affects Flags: N Z </p><pre>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 +</pre> +<p> +</p><p> +</p><p><a name="LSR"> </a> +</p><h2>LSR (Logical Shift Right) </h2> +<p>Affects Flags: N Z C </p><pre>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 + +</pre>LSR shifts all bits right one position. 0 is shifted into bit 7 and the +original bit 0 is shifted into the Carry. +<p> +</p><p> +</p><p><a name="WRAP"> </a> +</p><h2>Wrap-Around</h2> +<p>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. +</p><p> +It is possible, however, to access $017F when X = $FF by using the Absolute,X +addressing mode of LDA $80,X. That is, instead of: +</p><pre> LDA $80,X ; ZeroPage,X - the resulting object code is: B5 80 +</pre> +which accesses $007F when X=$FF, use: +<pre> LDA $0080,X ; Absolute,X - the resulting object code is: BD 80 00 +</pre> +which accesses $017F when X = $FF (a at cost of one additional byte and one +additional cycle). All of the ZeroPage,X and ZeroPage,Y instructions except +STX ZeroPage,Y and STY ZeroPage,X have a corresponding Absolute,X and +Absolute,Y instruction. Unfortunately, a lot of 6502 assemblers don't have an +easy way to force Absolute addressing, i.e. most will assemble a LDA $0080,X +as B5 80. One way to overcome this is to insert the bytes using the .BYTE +pseudo-op (on some 6502 assemblers this pseudo-op is called DB or DFB, +consult the assembler documentation) as follows: +<pre> .BYTE $BD,$80,$00 ; LDA $0080,X (absolute,X addressing mode) +</pre> +The comment is optional, but highly recommended for clarity. +<p>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. +</p><p><a name="PC"> </a> +</p><h2>Program Counter</h2> +<p>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). +</p><p>The program counter is loaded least signifigant byte first. Therefore the +most signifigant byte must be pushed first when creating a false return address. + +</p><p>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 4 bytes before the branch instruction. +</p><p><a name="TIMES"> </a> +</p><h2>Execution Times</h2> +<p>Op code execution times are measured in machine cycles; one machine cycle +equals one clock cycle. Many instructions require one extra cycle for +execution if a page boundary is crossed; these are indicated by a + following +the time values shown. +</p><p> +</p><p> +</p><p><a name="NOP"> </a> +</p><h2>NOP (No OPeration) </h2> +<p>Affects Flags: none </p><pre>MODE SYNTAX HEX LEN TIM +Implied NOP $EA 1 2 + +</pre>NOP is used to reserve space for future modifications or effectively REM +out existing code. +<p> +</p><p> +</p><p><a name="ORA"> </a> +</p><h2>ORA (bitwise OR with Accumulator) </h2> +<p>Affects Flags: N Z </p><pre>MODE SYNTAX HEX LEN TIM +Immediate ORA #$44 $09 2 2 +Zero Page ORA $44 $05 2 3 +Zero Page,X ORA $44,X $15 2 4 +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 +</pre> +<p> +</p><p> +</p><p><a name="TAX"> </a> <a name="TXA"> </a> <a name="TAY"> </a> <a name="TYA"> </a> <a name="INX"> </a> <a name="DEX"> </a> <a name="INY"> </a> <a name="DEY"> </a> +</p><h2>Register Instructions </h2> +<p>Affect Flags: N Z +</p><p>These instructions are implied mode, have a length of one byte and require +two machine cycles. </p><pre>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 +</pre> +<p> +</p><p> +</p><p><a name="ROL"> </a> +</p><h2>ROL (ROtate Left) </h2> +<p>Affects Flags: N Z C </p><pre>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 + +</pre>ROL shifts all bits left one position. The Carry is shifted into bit 0 and +the original bit 7 is shifted into the Carry. +<p> +</p><p> +</p><p><a name="ROR"> </a> +</p><h2>ROR (ROtate Right) </h2> +<p>Affects Flags: N Z C </p><pre>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 + +</pre>ROR shifts all bits right one position. The Carry is shifted into bit 7 +and the original bit 0 is shifted into the Carry. +<p> +</p><p> +</p><p><a name="RTI"> </a> +</p><h2>RTI (ReTurn from Interrupt) </h2> +<p>Affects Flags: all </p><pre>MODE SYNTAX HEX LEN TIM +Implied RTI $40 1 6 + +</pre>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). +<p>Note that unlike RTS, the return address on the stack is the actual address +rather than the address-1. +</p><p> +</p><p> +</p><p><a name="RTS"> </a> +</p><h2>RTS (ReTurn from Subroutine) </h2> +<p>Affects Flags: none </p><pre>MODE SYNTAX HEX LEN TIM +Implied RTS $60 1 6 + +</pre>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 <a href="http://www.6502.org/tutorials/6502opcodes.html#JSR">JSR</a> which +pushed the address-1. +<p>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: </p><pre> 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 +</pre> +<p> +</p><p><a name="SBC"> </a> +</p><h2>SBC (SuBtract with Carry)</h2> +<p>Affects Flags: N V Z C </p><pre>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 + +</pre>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). +<p>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. +</p><p> +</p><p> +</p><p><a name="STA"> </a> +</p><h2>STA (STore Accumulator) </h2> +<p>Affects Flags: none </p><pre>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 +</pre> +<p> +</p><p><a name="TXS"> </a> <a name="TSX"> </a> <a name="PHA"> </a> <a name="PLA"> </a> <a name="PHP"> </a> <a name="PLP"> </a> <a name="STACK"> </a> +</p><h2>Stack Instructions</h2> +<p>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. </p><pre>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 +</pre> +<p> +</p><p> +</p><p><a name="STX"> </a> +</p><h2>STX (STore X register)</h2> +<p>Affects Flags: none </p><pre>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 +</pre> +<p> +</p><p> +</p><p><a name="STY"> </a> +</p><h2>STY (STore Y register)</h2> +<p>Affects Flags: none </p><pre>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 +</pre> +<p> +</p><p><font size="-1">Last Updated Oct 17, 2020.</font> + +</p></body></html>
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