Direct Page and Stack at C2 Bank, and Other Fundamental Info

For the Final Fantasy 3 game, with a few exceptions, the developers didn't program with the direct page methodology. They set the direct page as zero and generally use it to clear the B register. (top byte of the A register in 8-bit mode).

If you need a example of direct page, you can check the following code in the assembly:

Code:

C2/286E: F4 00 11     PEA $1100         (Set direct page register 11 $1100)
C2/2871: 2B           PLD 
C2/2872: A5 C9        LDA $C9 ($11C9)
C2/2874: C9 9F        CMP #$9F          (Moogle Suit in character's Armor slot?)
C2/2876: D0 0B        BNE $2883         (if not, branch)

For this example, PEA will put the address in the stack (two bytes) and PLD will remove it from the stack and set it as the direct page address. Note that the direct page is a special register and there are other opcodes which can copy or setup it as well. Common opcodes related with the setting/copy of the direct page are TCD, TDC, PHD and PLD.

Opcodes related with the direct page has only one byte in the range [0, 255]. However, they are incremented with the direct page when they are executed. In this example, LDA $C9 is actually equivalent to LDA $11C9. The effective address is the direct page value ($1100) plus the operand ($C9), which is $11C9.

I never found any opcode in the game related with the relocation of the stack pointer. However, specific opcodes exist for the stack relocation. They are TCS, TSC, TXS and TSX.