But how instructions like DEC, DEX and DEY works ?
By adding $FF provided by the precharged internal data bus to the register content.
- The internal databus is precharged with $FF during PHI2 (*1)
- During the next phase it's loaded into the B register (signal DB/ADD)
- At the same time the index register is transfered via SB (*/SB) to the A register (SB/ADD)
- SUMS is pulled thus $FF is added to the register content, effectively decrementing it (*2).
- The result is transfered back via SB (ADD/SB7 + ADD/SB(0..6)) to either index register (SB/*).
Background 1: NMOS switching (Abbreviated Version)
While using the precharge to supply a $FF constant is a real nifty feature, it's more of a hack base on a side effect to speed up NMOS operation.
The precharge exists due the way MOS-FET and NMOS Logic work. Effectively each FET (and line) is a (comparable) large capacitor. Loading and unloading takes time in relation to resistance in the circuit. In NMOS logic the transistor is placed between the output and ground. On the other side the output is connected to Vcc (+5V) via a resistor.
When the Transistor switches on, the output is grounded with a very low resistance, puting the output extrem fast toward Low. When it's switched off, the output is loaded via the resistor. Thus changing a signal line from Low to High takes considerable longer than going from High to Low.
By precharging lines with High value ahead of time and then pulling them down when a signal is to be transmitted, minimizes switch time to the lower of both, resulting in faster operation.
Background 2: Use of Precharge for Constant Generation on 6502
On the 6502 precharge is thus present on all 4 internal busses (DB, ADL, ADH, SB). Beside providing $FF for decrement on DB, it's well used on ADH and ADL to generate constant values:
- Constant generated on DB
- Constants generated on ADH:
- $FF for vector pull - by default
- $01 for stack page addressing - by pulling 0/ADH(1-7)
- $00 for zero page addressing - by pulling 0/ADH(1-7) and 0/ADH0
- Constants generated on ADL:
- $FE/$FF for INT - by default value plus 0/ADL0 on first access
- $FC/$FD for Reset - by 0/ADL1 plus 0/ADL0 on first access
- $FA/$FB for NMI - by 0/ADL2 plus 0/ADL0 on first access
Another special use is with transfering ALU results to SB. The ALU does not shift-in Carry on a ROR, but always sets bit 7 to Zero. So when the ALU result is transfered after a ROR, bit 7 is only transfered if carry was cleared. Otherwise transfer for bit 7 is supressed, replacing it by a precharged One.
Precharge behaviour is a major reason why the 6502 is not static, as the precharged values will vanish after some time, making many operations invalid.
Background 3: Hanson Diagramm
It has been asked where the signal names originated. They are taken from the famous Hanson diagram - as cited by the OP (*3) - which is nowadays the de-facto standard for interpreting the 6502 (despite having a few little inconsistencies; *4). It was part of an equally famous 1995 article by Mr. Hanson about making D-Latches in VHDL. To my knowledge the diagram was first drawn up in 1982 during studies of the original 6502 blueprints.
The names are rather straight forward, like Y/SB meaning connect Y output to Special Bus (the register data bus), or SB/Y to enable loading Y from SB. Next to all modern sources use these names, or derivations thereof. Most notable maybe the Visual 6502 simulation (used in DPcontrol output; see the nodenames.js source for all names used; *5).
*1 - Note the Prechage MOSFET box right below the Data Output Register box
*2 - It's an unsigned 8 bit addition
(n + 255) mod 256.
*3 - Please use, whenever possible the cleaned up version provided by Mr. Hanson at http://www.witwright.com/DonPub/6502-Block-Diagram.pdf. The often used copies of copies of scans are quite ugly and hard to read.
*4 - For example there is a signal generated if DB is negative or the flipflops shown as part of the logic don't fully add up, but these are details that only come up when trying to recreate the CPU.
*5 - Searching it is quite handy to find meaning and usage - as the Visual 6502 in general is a great tool to peek into the 6502.