6502 flag manipulation
======================
The Carry flag can be set cleared with SEC and CLC, but the other flags have
no instructions to do so.

oVerflow flag
-------------
The 6502 has a CLV instruction to clear the oVerflow flag, but has no
complementary SEV instruction to set the V flag. However, with careful 
arranging of code, you can do this by testing a byte of the code.

For example:

  BIT setv   ; V set from b6 of RTS opcode (also clears M)
  .setv
  RTS        ; opcode is %01100000

  BIT setv   ; V set from b6 of JMP opcode (also clears M)
  .setv
  JMP dest   ; opcode is %01001100

  BIT setv   ; V set from b6 of JMP opcode (also clears M)
  .setv
  JMP (vect) ; opcode is %01101100

  BIT setv   ; V set from b6 of NOP opcode (also sets M)
  .setv
  NOP        ; opcode is %11001010

  .bit
  LDA &FE08  ; where absaddr>=&C000
  BIT bit+2  ; V set from b14 of &FE08 (also sets M)

  .bit
  LDA &FD    ; where zpaddr>=&C0
  BIT bit+1  ; V set from b6 of &FD (also sets M)


Minus flag
----------
  .bit
  LDA &FE08  ; where absaddr>=&8000
  BIT bit+2  ; sets M from bit 15 &FE08

  .bit
  LDA &FD    ; where zpaddr>=&80
  BIT bit+1  ; sets M from bit 7 of &FD

  ORA #&80   ; sets M, changing A

  LDr #num   ; if num>&7F, sets M changing register

  .clrm
  BIT clrm   ; opcode is %00101100, clears M

  AND #&7F   ; clears M, changing A

  LDr #num   ; if num<&80, clears M changing register


Zero flag
---------
  CMP #num   ; if num known to be same as A, will set Z
  CMP #num   ; if num known to be different from A, will clear Z
  LDr #0     ; sets Z, also clearing register


Complement Carry
----------------
  PHP:PLA:EOR #&01:PHA:PLP

  ROL A:EOR #&01:ROR A   ; also modifies M and Z

  ROR A:EOR #&80:ROL A   ; also modifies M and Z