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Why does the 6502 have zero page opcodes?
The use of zero or zero page values will result in assembled code with zero page opcodes when you wanted absolute codes. With $FFFF, the problem is in addresses+1 as you wrap around to page 0. When the 6502 is ready for the next instruction it increments the program counter before fetching the instruction.
How to transfer control to$ 4080 using 6502?
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. Affects Flags: none
Can you add a register to the accumulator in 6502?
I’ve just started playing around with some 6502 development on the Atari 2600 for the first time, and it appears as though you can’t add the value of the X or Y registers to the accumulator. Think of it like the 68k’s distinction between data and address registers.
What is the address of the 6502 instruction?
6502 instruction operation codes (opcodes) are eight-bits long and have the general form AAABBBCC, where AAA and CC define the opcode, and BBB defines the addressing mode. [70] For instance, consider the ORA instruction, which performs a bitwise OR on the bits in the accumulator with another value.
When does the 6502 increment the 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.
How does a 6502 assembly language statement work?
A 6502 assembly language statement consists of a three-character instruction mnemonic, followed by any operands. Instructions that do not take a separate operand but target a single register based on the addressing mode combine the target register in the instruction mnemonic, so the assembler uses INX as opposed to INC X to increment the X
What does the interrupt flag do on the 6502?
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.