All Questions
Tagged with 6502design-choices
5 questions
9votes
1answer
565views
What is the purpose of the 2x4-bit split of the PCH incrementer?
On this detailed 6502 block diagram that can be found all over the net, the increment logic for the high byte of the program counter is split into two 4-bit parts, with a named PCHC line for the carry ...
- 2,820
10votes
2answers
3kviews
How did the 6502 designers intend for the indirect addressing modes to be used?
The 6502 has three indirect addressing modes: Indirect, Indexed Indirect (Indirect,X) and Indirect Indexed (Indirect,Y). Indirect is only used on the JMP instruction, no other. Why was this mode ...
- 1,183
25votes
2answers
4kviews
Was leaving all xxxxxx11 opcodes unused on the 6502 a deliberate design choice?
The 6502, like many 8-bit processors, has a somewhat arcane opcode-mode restrictions. On most such processors, the restriction is a clear result of trying to pack a lot of instructions into a limited ...
- 40.2k
19votes
2answers
4kviews
Why does the 6502 JSR instruction only increment the return address by 2 bytes?
Currently messing with 6502 assembly on a C64, and I don't understand why the JSR instruction is so weird. According to the instruction table, JSR is a 3-byte instruction and only operates in absolute ...
22votes
1answer
2kviews
Why did the 6502 handle BCD with a special mode?
The 6502 had special support for BCD arithmetic, because it was widely used in those days; this much, it had in common with other CPUs. But the 8080 and 6800 implemented this in the form of a 'decimal ...
- 64.5k
