6502 OPCODES PDF

Wrap-Around Use caution with indexed zero page operations as they are subject to wrap-around. 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. The use of zero or zero page values will result in assembled code with zero page opcodes when you wanted absolute codes. Program Counter When the 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.

Author:Kizshura Shakamuro
Country:Belgium
Language:English (Spanish)
Genre:Business
Published (Last):21 November 2004
Pages:236
PDF File Size:17.86 Mb
ePub File Size:13.48 Mb
ISBN:306-5-39693-785-6
Downloads:79658
Price:Free* [*Free Regsitration Required]
Uploader:Daigrel



The address of the branch destination LABEL in the syntax example above must be within to bytes inclusive of the address of the next instruction or data after the BRA, that is, 2 bytes plus the address of the BRA opcode. The branch is to a different page when the next instruction after the BRA is on a different page than the branch destination. Note that this is the same cycle count as the other branch instructions. Since the other branches are conditional, they will take 2 cycles if the branch is not taken i.

PHX 5A 1 3 imp Note that the cycle counts are the same as STA. First, the term reset is used to refer to the clearing of a bit, whereas the term clear had been used consistently before, such as CLC which stands for CLear Carry. Second, the effect on the Z flag is determined by a different function than the effect on memory.

Specifically, it is based on whether the result of a bitwise AND of the accumulator with the contents of the memory location specified in the operand is zero. Also, like BIT, the accumulator is not affected. The accumulator determines which bits in the memory location specified in the operand are cleared and which are not affected. The bits in the accumulator that are ones are cleared in memory , and the bits that are zeros in the accumulator are not affected in memory.

This is the same as saying that the resulting memory contents are the bitwise AND of the memory contents with the complement of the accumulator i. Here are a couple of examples to help illustrate the operation of TRB. Incidentally, when the Z flag is 1 after a TRB instruction , the memory location will have the same value it did before the TRB instruction. The reason this is true is that for the Z flag to be 1, the bits in the accumulator that are ones must be zeros in the memory location.

For one, like TRB, the effect on the Z flag is determined by a different function than the effect on memory. The accumulator determines which bits in the memory location specified in the operand are set and which are not affected. The bits in the accumulator that are ones are set to one in memory , and the bits that are zeros in the accumulator are not affected in memory. This is the same as saying that the resulting memory contents are the bitwise OR of the memory contents with the accumulator.

Here are a couple of examples to help illustrate the operation of TSB. Four additional instructions and one additional addressing mode are available on 65C02s manufactured by Rockwell and WDC. Unlike other branch instructions, BBR and BBS always take the same number of cycles five whether the branch is taken or not. It is often useful to test bit 0, for example, to test whether a byte is even or odd. First, there is only a single addressing mode for these instructions -- no indexing by X or Y, for instance.

Second, they are restricted to zero page locations. The addressing mode is a combination of zero page addressing and relative addressing -- really just a juxtaposition of the two. The bit to test is typically specified as part of the instruction name rather than the operand, i.

First, the zero page address comes before the branch destination address label , which is the same order as the object code. The second byte of the instruction is the zero page address, and the third byte is the signed branch displacement. Second, although less common, the latter syntax can be more flexible if an assembler allows the bit to test in the example, 0 to be a numeric expression.

This would allow the bit number to be specified in an EQUate. However, it is worth noting that it is rarely useful to preserve the value of the Z zero flag the only flag affected by TRB and TSB , unlike other flags such as the carry. Two additional instructions i. This puts the 65C02 into a low power state. This is useful for applications circuits that require low power consumption, but STP is rarely seen otherwise. In addition to reducing power consumption, using WAI also ensures that the interrupt will be recognized immediately.

In other words, if an interrupt e. ADC , the instruction must finish before the interrupt will be recognized i. When WAI is used, once its third cycle is complete, the 65C02 will wait for the interrupt and can respond to it without any additional delay whenever it occurs.

The above is true of an IRQ when the I interrupt disable flag is clear i. In this case, when an IRQ occurs after the WAI instruction , the 65C02 will continue with with the next instruction rather than jumping to the interrupt vector. This means an IRQ can be responded to within one cycle! The interrupt handler is effectively inline code, rather than a separate routine, and thus it does not end with an RTI, resulting in fewer cycles needed to handle the interrupt.

Instructions with functional differences The BRK instruction is the only instruction on the 65C02 that has a functional difference from the , but the same cycle count.

On both, the value of processor status P register is pushed onto the stack after the high and low bytes of the return address have been pushed with bit 4 the B "flag" set i. When an IRQ occurs, the processor status register is pushed onto the stack again, after the return address has been pushed with bit 4 clear. Thus it is not necessary for any interrupt handler to use a CLD instruction to put the D flag in a known state. This is not such a big deal with RESET handlers, but saving two cycles the CLD instruction in interrupt handlers can be significant, since interrupt handlers often are usually intended to be as fast as possible.

Four instructions are functionally identical on the 65C02 and , but have different cycle counts. On the 65C02, they take 6 cycles when a page boundary is not crossed, and take 7 cycles when a page boundary is crossed. The instructions are otherwise the same on the and 65C In decimal mode i. The main functional difference is that the N, V, and Z flag results are valid in addition to the accumulator and C flag result on the 65C On the , only the accumulator and C flag results were valid.

As it turns out the V flag result is the same on the , 65C02, and in decimal mode. Finally, in the realm of undocumented behavior, there are instances where the and 65C02 will have different accumulator results in decimal mode when there is an invalid BCD number.

Consequently, a JMP abs takes 6 cycles on the 65C02, whereas it took only 5 cycles on the Unused opcodes guaranteed NOPs On the , with the valid combinations the instructions and their addressing modes, of the possible opcodes were used. The remaining opcodes were not handled specially, but were just simply fed to the internal decoding logic and executed. On the 65C02, all unused opcodes are guaranteed to have no operation, and are documented as such. It is documented as having no operation, but is reserved for future instruction set expansion.

The following table summarizes the unused opcodes of the 65C The first number is the size in bytes, and the second number is the number of cycles taken.

After the second number, a lower case letter may be present; when it is present it indicates a footnote. Note, however, that any code that makes use of them is limited to the 65C First, many opcodes behave as one byte, one cycle NOPs.

Since BCS does not affect any flags, it serves, in this situation, as a two byte, two cycle NOP and provides a subtle, but useful way to efficiently skip the SEC instruction.

This has a few advantages. First, the next byte will be skipped regardless of the flag values. Second, it does not affect any flags or registers. Third, it will only take two cycles, which is the fastest any two byte instruction can take.

DM9161AEP DATASHEET PDF

6502 Unsupported Opcodes

EA Instruction timing The time required for instruction to execute is regular and predictable. The primary rule is this: Each byte read from or written to memory requires one clock cycle. All single-byte instructions waste a cycle reading and ignoring the byte that comes immediately after the instruction this means no instruction can take less than two cycles. Zero page,X, zero page,Y, and zero page,X addressing modes spend an extra cycle reading the unindexed zero page address. Absolute,X, absolute,Y, and zero page ,Y addressing modes need an extra cycle if the indexing crosses a page boundary, or if the instruction writes to memory.

PNOZ S3 PDF

6502 all 256 Opcodes

The address of the branch destination LABEL in the syntax example above must be within to bytes inclusive of the address of the next instruction or data after the BRA, that is, 2 bytes plus the address of the BRA opcode. The branch is to a different page when the next instruction after the BRA is on a different page than the branch destination. Note that this is the same cycle count as the other branch instructions. Since the other branches are conditional, they will take 2 cycles if the branch is not taken i.

Related Articles