Documentation for the NMOS 65xx/85xx Instruction Set 6510 Instructions by Addressing Modes 6502 Registers 6510/8502 Undocumented Commands Register selection for load and store Decimal mode in NMOS 6500 series 6510 features Different CPU types 6510 Instruction Timing How Real Programmers Acknowledge Interrupts Memory Management Autostart Code Notes References 6510 Instructions by Addressing Modes off- ++++++++++ Positive ++++++++++ ---------- Negative ---------- set 00 20 40 60 80 a0 c0 e0 mode +00 BRK JSR RTI RTS NOP* LDY CPY CPX Impl/immed +01 ORA AND EOR ADC STA LDA CMP SBC (indir,x) +02 t t t t NOP*t LDX NOP*t NOP*t ? /immed +03 SLO* RLA* SRE* RRA* SAX* LAX* DCP* ISB* (indir,x) +04 NOP* BIT NOP* NOP* STY LDY CPY CPX Zeropage +05 ORA AND EOR ADC STA LDA CMP SBC Zeropage +06 ASL ROL LSR ROR STX LDX DEC INC Zeropage +07 SLO* RLA* SRE* RRA* SAX* LAX* DCP* ISB* Zeropage +08 PHP PLP PHA PLA DEY TAY INY INX Implied +09 ORA AND EOR ADC NOP* LDA CMP SBC Immediate +0a ASL ROL LSR ROR TXA TAX DEX NOP Accu/impl +0b ANC** ANC** ASR** ARR** ANE** LXA** SBX** SBC* Immediate +0c NOP* BIT JMP JMP () STY LDY CPY CPX Absolute +0d ORA AND EOR ADC STA LDA CMP SBC Absolute +0e ASL ROL LSR ROR STX LDX DEC INC Absolute +0f SLO* RLA* SRE* RRA* SAX* LAX* DCP* ISB* Absolute +10 BPL BMI BVC BVS BCC BCS BNE BEQ Relative +11 ORA AND EOR ADC STA LDA CMP SBC (indir),y +12 t t t t t t t t ? +13 SLO* RLA* SRE* RRA* SHA** LAX* DCP* ISB* (indir),y +14 NOP* NOP* NOP* NOP* STY LDY NOP* NOP* Zeropage,x +15 ORA AND EOR ADC STA LDA CMP SBC Zeropage,x +16 ASL ROL LSR ROR STX y) LDX y) DEC INC Zeropage,x +17 SLO* RLA* SRE* RRA* SAX* y) LAX* y) DCP* ISB* Zeropage,x +18 CLC SEC CLI SEI TYA CLV CLD SED Implied +19 ORA AND EOR ADC STA LDA CMP SBC Absolute,y +1a NOP* NOP* NOP* NOP* TXS TSX NOP* NOP* Implied +1b SLO* RLA* SRE* RRA* SHS** LAS** DCP* ISB* Absolute,y +1c NOP* NOP* NOP* NOP* SHY** LDY NOP* NOP* Absolute,x +1d ORA AND EOR ADC STA LDA CMP SBC Absolute,x +1e ASL ROL LSR ROR SHX**y) LDX y) DEC INC Absolute,x +1f SLO* RLA* SRE* RRA* SHA**y) LAX* y) DCP* ISB* Absolute,x ROR intruction is available on MC650x microprocessors after June, 1976. Legend: t Jams the machine *t Jams very rarely * Undocumented command ** Unusual operation y) indexed using Y instead of X () indirect instead of absolute Note that the NOP instructions do have other addressing modes than the implied addressing. The NOP instruction is just like any other load instruction, except it does not store the result anywhere nor affects the flags. 6502 Registers The NMOS 65xx processors are not ruined with too many registers. In addition to that, the registers are mostly 8-bit. Here is a brief description of each register: PC Program Counter This register points the address from which the next instruction byte (opcode or parameter) will be fetched. Unlike other registers, this one is 16 bits in length. The low and high 8-bit halves of the register are called PCL and PCH, respectively. The Program Counter may be read by pushing its value on the stack. This can be done either by jumping to a subroutine or by causing an interrupt. S Stack pointer The NMOS 65xx processors have 256 bytes of stack memory, ranging from $0100 to $01FF. The S register is a 8-bit offset to the stack page. In other words, whenever anything is being pushed on the stack, it will be stored to the address $0100+S. The Stack pointer can be read and written by transfering its value to or from the index register X (see below) with the TSX and TXS instructions. P Processor status This 8-bit register stores the state of the processor. The bits in this register are called flags. Most of the flags have something to do with arithmetic operations. The P register can be read by pushing it on the stack (with PHP or by causing an interrupt). If you only need to read one flag, you can use the branch instructions. Setting the flags is possible by pulling the P register from stack or by using the flag set or clear instructions. Following is a list of the flags, starting from the 8th bit of the P register (bit 7, value $80): N Negative flag This flag will be set after any arithmetic operations (when any of the registers A, X or Y is being loaded with a value). Generally, the N flag will be copied from the topmost bit of the register being loaded. Note that TXS (Transfer X to S) is not an arithmetic operation. Also note that the BIT instruction affects the Negative flag just like arithmetic operations. Finally, the Negative flag behaves differently in Decimal operations (see description below). V oVerflow flag Like the Negative flag, this flag is intended to be used with 8-bit signed integer numbers. The flag will be affected by addition and subtraction, the instructions PLP, CLV and BIT, and the hardware signal -SO. Note that there is no SEV instruction, even though the MOS engineers loved to use East European abbreviations, like DDR (Deutsche Demokratische Republik vs. Data Direction Register). (The Russian abbreviation for their former trade association COMECON is SEV.) The -SO (Set Overflow) signal is available on some processors, at least the 6502, to set the V flag. This enables response to an I/O activity in equal or less than three clock cycles when using a BVC instruction branching to itself ($50 $FE). The CLV instruction clears the V flag, and the PLP and BIT instructions copy the flag value from the bit 6 of the topmost stack entry or from memory. After a binary addition or subtraction, the V flag will be set on a sign overflow, cleared otherwise. What is a sign overflow? For instance, if you are trying to add 123 and 45 together, the result (168) does not fit in a 8-bit signed integer (upper limit 127 and lower limit -128). Similarly, adding -123 to -45 causes the overflow, just like subtracting -45 from 123 or 123 from -45 would do. Like the N flag, the V flag will not be set as expected in the Decimal mode. Later in this document is a precise operation description. A common misbelief is that the V flag could only be set by arithmetic operations, not cleared. 1 unused flag To the current knowledge, this flag is always 1. B Break flag This flag is used to distinguish software (BRK) interrupts from hardware interrupts (IRQ or NMI). The B flag is always set except when the P register is being pushed on stack when jumping to an interrupt routine to process only a hardware interrupt. The official NMOS 65xx documentation claims that the BRK instruction could only cause a jump to the IRQ vector ($FFFE). However, if an NMI interrupt occurs while executing a BRK instruction, the processor will jump to the NMI vector ($FFFA), and the P register will be pushed on the stack with the B flag set. D Decimal mode flag This flag is used to select the (Binary Coded) Decimal mode for addition and subtraction. In most applications, the flag is zero. The Decimal mode has many oddities, and it operates differently on CMOS processors. See the description of the ADC, SBC and ARR instructions below. I Interrupt disable flag This flag can be used to prevent the processor from jumping to the IRQ handler vector ($FFFE) whenever the hardware line -IRQ is active. The flag will be automatically set after taking an interrupt, so that the processor would not keep jumping to the interrupt routine if the -IRQ signal remains low for several clock cycles. Z Zero flag The Zero flag will be affected in the same cases than the Negative flag. Generally, it will be set if an arithmetic register is being loaded with the value zero, and cleared otherwise. The flag will behave differently in Decimal operations. C Carry flag This flag is used in additions, subtractions, comparisons and bit rotations. In additions and subtractions, it acts as a 9th bit and lets you to chain operations to calculate with bigger than 8-bit numbers. When subtracting, the Carry flag is the negative of Borrow: if an overflow occurs, the flag will be clear, otherwise set. Comparisons are a special case of subtraction: they assume Carry flag set and Decimal flag clear, and do not store the result of the subtraction anywhere. There are four kinds of bit rotations. All of them store the bit that is being rotated off to the Carry flag. The left shifting instructions are ROL and ASL. ROL copies the initial Carry flag to the lowmost bit of the byte; ASL always clears it. Similarly, the ROR and LSR instructions shift to the right. A Accumulator The accumulator is the main register for arithmetic and logic operations. Unlike the index registers X and Y, it has a direct connection to the Arithmetic and Logic Unit (ALU). This is why many operations are only available for the accumulator, not the index registers. X Index register X This is the main register for addressing data with indices. It has a special addressing mode, indexed indirect, which lets you to have a vector table on the zero page. Y Index register Y The Y register has the least operations available. On the other hand, only it has the indirect indexed addressing mode that enables access to any memory place without having to use self-modifying code. 6510/8502 Undocumented Commands -- A brief explanation about what may happen while using don't care states. ANE $8B A = (A | #$EE) & X & #byte same as A = ((A & #$11 & X) | ( #$EE & X)) & #byte In real 6510/8502 the internal parameter #$11 may occasionally be #$10, #$01 or even #$00. This occurs when the video chip starts DMA between the opcode fetch and the parameter fetch of the instruction. The value probably depends on the data that was left on the bus by the VIC-II. LXA $AB C=Lehti: A = X = ANE Alternate: A = X = (A & #byte) TXA and TAX have to be responsible for these. SHA $93,$9F Store (A & X & (ADDR_HI + 1)) SHX $9E Store (X & (ADDR_HI + 1)) SHY $9C Store (Y & (ADDR_HI + 1)) SHS $9B SHA and TXS, where X is replaced by (A & X). Note: The value to be stored is copied also to ADDR_HI if page boundary is crossed. SBX $CB Carry and Decimal flags are ignored but the Carry flag will be set in substraction. This is due to the CMP command, which is executed instead of the real SBC. ARR $6B This instruction first performs an AND between the accumulator and the immediate parameter, then it shifts the accumulator to the right. However, this is not the whole truth. See the description below. Many undocumented commands do not use AND between registers, the CPU just throws the bytes to a bus simultaneously and lets the open-collector drivers perform the AND. I.e. the command called 'SAX', which is in the STORE section (opcodes $A0...$BF), stores the result of (A & X) by this way. More fortunate is its opposite, 'LAX' which just loads a byte simultaneously into both A and X. $6B ARR This instruction seems to be a harmless combination of AND and ROR at first sight, but it turns out that it affects the V flag and also has a special kind of decimal mode. This is because the instruction has inherited some properties of the ADC instruction ($69) in addition to the ROR ($6A). In Binary mode (D flag clear), the instruction effectively does an AND between the accumulator and the immediate parameter, and then shifts the accumulator to the right, copying the C flag to the 8th bit. It sets the Negative and Zero flags just like the ROR would. The ADC code shows up in the Carry and oVerflow flags. The C flag will be copied from the bit 6 of the result (which doesn't seem too logical), and the V flag is the result of an Exclusive OR operation between the bit 6 and the bit 5 of the result. This makes sense, since the V flag will be normally set by an Exclusive OR, too. In Decimal mode (D flag set), the ARR instruction first performs the AND and ROR, just like in Binary mode. The N flag will be copied from the initial C flag, and the Z flag will be set according to the ROR result, as expected. The V flag will be set if the bit 6 of the accumulator changed its state between the AND and the ROR, cleared otherwise. Now comes the funny part. If the low nybble of the AND result, incremented by its lowmost bit, is greater than 5, the low nybble in the ROR result will be incremented by 6. The low nybble may overflow as a consequence of this BCD fixup, but the high nybble won't be adjusted. The high nybble will be BCD fixed in a similar way. If the high nybble of the AND result, incremented by its lowmost bit, is greater than 5, the high nybble in the ROR result will be incremented by 6, and the Carry flag will be set. Otherwise the C flag will be cleared. To help you understand this description, here is a C routine that illustrates the ARR operation in Decimal mode: unsigned A, /* Accumulator */ AL, /* low nybble of accumulator */ AH, /* high nybble of accumulator */ C, /* Carry flag */ Z, /* Zero flag */ V, /* oVerflow flag */ N, /* Negative flag */ t, /* temporary value */ s; /* value to be ARRed with Accumulator */ t = A & s; /* Perform the AND. */ AH = t >> 4; /* Separate the high */ AL = t & 15; /* and low nybbles. */ N = C; /* Set the N and */ Z = !(A = (t >> 1) | (C << 7)); /* Z flags traditionally */ V = (t ^ A) & 64; /* and V flag in a weird way. */ if (AL + (AL & 1) > 5) /* BCD "fixup" for low nybble. */ A = (A & 0xF0) | ((A + 6) & 0xF); if (C = AH + (AH & 1) > 5) /* Set the Carry flag. */ A = (A + 0x60) & 0xFF; /* BCD "fixup" for high nybble. */ $CB SBX X <- (A & X) - Immediate The 'SBX' ($CB) may seem to be very complex operation, even though it is a combination of the subtraction of accumulator and parameter, as in the 'CMP' instruction, and the command 'DEX'. As a result, both A and X are connected to ALU but only the subtraction takes place. Since the comparison logic was used, the result of subtraction should be normally ignored, but the 'DEX' now happily stores to X the value of (A & X) - Immediate. That is why this instruction does not have any decimal mode, and it does not affect the V flag. Also Carry flag will be ignored in the subtraction but set according to the result. Proof: begin 644 vsbx M`0@9$,D'GL(H-#,IJC(U-JS"*#0T*:HR-@```*D`H#V1*Z`_D2N@09$KJ0>% M^QBE^VEZJ+$KH#F1*ZD`2"BI`*(`RP`(:-B@.5$K*4#P`E@`H#VQ*SAI`)$K JD-Z@/[$K:0"1*Y#4J2X@TO\XH$&Q*VD`D2N0Q,;[$+188/_^]_:_OK>V ` end and begin 644 sbx M`0@9$,D'GL(H-#,IJC(U-JS"*#0T*:HR-@```'BI`*!-D2N@3Y$KH%&1*ZD# MA?L8I?M*2)`#J1@LJ3B@29$K:$J0`ZGX+*G8R)$K&/BXJ?2B8\L)AOP(:(7] MV#B@3;$KH$\Q*Z!1\2L(1?SP`0!H1?TIM]#XH$VQ*SAI`)$KD,N@3[$K:0"1 9*Y#!J2X@TO\XH%&Q*VD`D2N0L<;[$))88-#X ` end These test programs show if your machine is compatible with ours regarding the opcode $CB. The first test, vsbx, proves that SBX does not affect the V flag. The latter one, sbx, proves the rest of our theory. The vsbx test tests 33554432 SBX combinations (16777216 different A, X and Immediate combinations, and two different V flag states), and the sbx test doubles that amount (16777216*4 D and C flag combinations). Both tests have run successfully on a C64 and a Vic20. They ought to run on C16, +4 and the PET series as well. The tests stop with BRK, if the opcode $CB does not work as expected. Successful operation ends in RTS. As the tests are very slow, they print dots on the screen while running so that you know that the machine has not jammed. On computers running at 1 MHz, the first test prints approximately one dot every four seconds and a total of 2048 dots, whereas the second one prints half that amount, one dot every seven seconds. If the tests fail on your machine, please let us know your processor's part number and revision. If possible, save the executable (after it has stopped with BRK) under another name and send it to us so that we know at which stage the program stopped. The following program is a Commodore 64 executable that Marko M"akel"a developed when trying to find out how the V flag is affected by SBX. (It was believed that the SBX affects the flag in a weird way, and this program shows how SBX sets the flag differently from SBC.) You may find the subroutine at $C150 useful when researching other undocumented instructions' flags. Run the program in a machine language monitor, as it makes use of the BRK instruction. The result tables will be written on pages $C2 and $C3. begin 644 sbx-c100 M`,%XH`",#L&,$,&,$L&XJ8*B@LL7AOL(:(7\N#BM#L$M$,'M$L$(Q?OP`B@` M:$7\\`,@4,'N#L'0U.X0P=#/SB#0[A+!T,<``````````````)BJ\!>M#L$M L$,'=_\'0":T2P=W_PM`!8,K0Z:T.P2T0P9D`PID`!*T2P9D`PYD`!HL2N@ M3Y$KH%R1*XII>ZBQ*Z!3D2N@8)$KBFE_J+$KH%61*Z!BD2OX.+BE^^;\Q_S8 L"&B%_3BXI?OF_,?\"&A%_?`!`.;[T-_F_-#;RA"M8!@X&#CFYL;&Q\?GYP#8 ` end 6510 features o PHP always pushes the Break (B) flag as a `1' to the stack. Jukka Tapanim"aki claimed in C=lehti issue 3/89, on page 27 that the processor makes a logical OR between the status register's bit 4 and the bit 8 of the stack pointer register (which is always 1). He did not give any reasons for this argument, and has refused to clarify it afterwards. Well, this was not the only error in his article... o Indirect addressing modes do not handle page boundary crossing at all. When the parameter's low byte is $FF, the effective address wraps around and the CPU fetches high byte from $xx00 instead of $xx00+$0100. E.g. JMP ($01FF) fetches PCL from $01FF and PCH from $0100, and LDA ($FF),Y fetches the base address from $FF and $00. o Indexed zero page addressing modes never fix the page address on crossing the zero page boundary. E.g. LDX #$01 : LDA ($FF,X) loads the effective address from $00 and $01. o The processor always fetches the byte following a relative branch instruction. If the branch is taken, the processor reads then the opcode from the destination address. If page boundary is crossed, it first reads a byte from the old page from a location that is bigger or smaller than the correct address by one page. o If you cross a page boundary in any other indexed mode, the processor reads an incorrect location first, a location that is smaller by one page. o Read-Modify-Write instructions write unmodified data, then modified (so INC effectively does LDX loc;STX loc;INX;STX loc) o -RDY is ignored during writes (This is why you must wait 3 cycles before doing any DMA -- the maximum number of consecutive writes is 3, which occurs during interrupts except -RESET.) o Some undefined opcodes may give really unpredictable results. o All registers except the Program Counter remain unmodified after -RESET. (This is why you must preset D and I flags in the RESET handler.) Different CPU types The Rockwell data booklet 29651N52 (technical information about R65C00 microprocessors, dated October 1984), lists the following differences between NMOS R6502 microprocessor and CMOS R65C00 family: 1. Indexed addressing across page boundary. NMOS: Extra read of invalid address. CMOS: Extra read of last instruction byte. 2. Execution of invalid op codes. NMOS: Some terminate only by reset. Results are undefined. CMOS: All are NOPs (reserved for future use). 3. Jump indirect, operand = XXFF. NMOS: Page address does not increment. CMOS: Page address increments and adds one additional cycle. 4. Read/modify/write instructions at effective address. NMOS: One read and two write cycles. CMOS: Two read and one write cycle. 5. Decimal flag. NMOS: Indeterminate after reset. CMOS: Initialized to binary mode (D=0) after reset and interrupts. 6. Flags after decimal operation. NMOS: Invalid N, V and Z flags. CMOS: Valid flag adds one additional cycle. 7. Interrupt after fetch of BRK instruction. NMOS: Interrupt vector is loaded, BRK vector is ignored. CMOS: BRK is executed, then interrupt is executed. 6510 Instruction Timing The NMOS 6500 series processors always perform at least two reads for each instruction. In addition to the operation code (opcode), they fetch the next byte. This is quite efficient, as most instructions are two or three bytes long. The processors also use a sort of pipelining. If an instruction does not store data in memory on its last cycle, the processor can fetch the opcode of the next instruction while executing the last cycle. For instance, the instruction EOR #$FF truly takes three cycles. On the first cycle, the opcode $49 will be fetched. During the second cycle the processor decodes the opcode and fetches the parameter #$FF. On the third cycle, the processor will perform the operation and store the result to accumulator, but simultaneously it fetches the opcode for the next instruction. This is why the instruction effectively takes only two cycles. The following tables show what happens on the bus while executing different kinds of instructions. Interrupts NMI and IRQ both take 7 cycles. Their timing diagram is much like BRK's (see below). IRQ will be executed only when the I flag is clear. IRQ and BRK both set the I flag, whereas the NMI does not affect its state. The processor will usually wait for the current instruction to complete before executing the interrupt sequence. To process the interrupt before the next instruction, the interrupt must occur before the last cycle of the current instruction. There is one exception to this rule: the BRK instruction. If a hardware interrupt (NMI or IRQ) occurs before the fourth (flags saving) cycle of BRK, the BRK instruction will be skipped, and the processor will jump to the hardware interrupt vector. This sequence will always take 7 cycles. You do not completely lose the BRK interrupt, the B flag will be set in the pushed status register if a BRK instruction gets interrupted. When BRK and IRQ occur at the same time, this does not cause any problems, as your program will consider it as a BRK, and the IRQ would occur again after the processor returned from your BRK routine, unless you cleared the interrupt source in your BRK handler. But the simultaneous occurrence of NMI and BRK is far more fatal. If you do not check the B flag in the NMI routine and subtract two from the return address when needed, the BRK instruction will be skipped. If the NMI and IRQ interrupts overlap each other (one interrupt occurs before fetching the interrupt vector for the other interrupt), the processor will most probably jump to the NMI vector in every case, and then jump to the IRQ vector after processing the first instruction of the NMI handler. This has not been measured yet, but the IRQ is very similar to BRK, and many sources state that the NMI has higher priority than IRQ. However, it might be that the processor takes the interrupt that comes later, i.e. you could lose an NMI interrupt if an IRQ occurred in four cycles after it. After finishing the interrupt sequence, the processor will start to execute the first instruction of the interrupt routine. This proves that the processor uses a sort of pipelining: it finishes the current instruction (or interrupt sequence) while reading the opcode of the next instruction. RESET does not push program counter on stack, and it lasts probably 6 cycles after deactivating the signal. Like NMI, RESET preserves all registers except PC. Instructions accessing the stack BRK # address R/W description --- ------- --- ----------------------------------------------- 1 PC R fetch opcode, increment PC 2 PC R read next instruction byte (and throw it away), increment PC 3 $0100,S W push PCH on stack (with B flag set), decrement S 4 $0100,S W push PCL on stack, decrement S 5 $0100,S W push P on stack, decrement S 6 $FFFE R fetch PCL 7 $FFFF R fetch PCH RTI # address R/W description --- ------- --- ----------------------------------------------- 1 PC R fetch opcode, increment PC 2 PC R read next instruction byte (and throw it away) 3 $0100,S R increment S 4 $0100,S R pull P from stack, increment S 5 $0100,S R pull PCL from stack, increment S 6 $0100,S R pull PCH from stack RTS # address R/W description --- ------- --- ----------------------------------------------- 1 PC R fetch opcode, increment PC 2 PC R read next instruction byte (and throw it away) 3 $0100,S R increment S 4 $0100,S R pull PCL from stack, increment S 5 $0100,S R pull PCH from stack 6 PC R increment PC PHA, PHP # address R/W description --- ------- --- ----------------------------------------------- 1 PC R fetch opcode, increment PC 2 PC R read next instruction byte (and throw it away) 3 $0100,S W push register on stack, decrement S PLA, PLP # address R/W description --- ------- --- ----------------------------------------------- 1 PC R fetch opcode, increment PC 2 PC R read next instruction byte (and throw it away) 3 $0100,S R increment S 4 $0100,S R pull register from stack JSR # address R/W description --- ------- --- ------------------------------------------------- 1 PC R fetch opcode, increment PC 2 PC R fetch low address byte, increment PC 3 $0100,S R internal operation (predecrement S?) 4 $0100,S W push PCH on stack, decrement S 5 $0100,S W push PCL on stack, decrement S 6 PC R copy low address byte to PCL, fetch high address