Zanac (NES)/RAM map

A preliminary and tentative list of RAM locations and their usage during gameplay. Regions are listed as address, size, then description.

  F  1 Currently swapped-in bank. This is set by the bank-swapping
       routines and is consulted by routines that push it onto the
       stack and restore it later.
 26  1 ROM bank holding compressed data.
 2B  1 Decompression control byte.
 2C  2 Pointer to current position in compressed data
 32  1 Lives. 0 = Game Over, any value up to 255 is valid
 33  1 Upgrade level of basic gun, from 0 (two single shots) through 5 (three double shots)
 34  1 Last direction pressed while Zanac is pilotable (From 0-8, SW, W, NW, S, Neutral, N, SE, E, NE)
 35  1 Basic gun stat: number of shots allowed on-screen at once
 36  1 Basic gun type: 1-3 for single/double/triple shot, 4 for supergun
 37  1 Basic gun bullet speed, ranging from -5 to -11
 51  1 Autofire delay on the basic gun. Starts at $0F, but can be dropped all the way to 1 (a new bullet every frame!)
 55  1 Total number of basic shots fired, mod 256
 56  1 Number of basic shots fired without scoring a kill, capped at 255
 5C  1 Current weapon, 0-7
 5E  1 Weapon timer/ammo. Defaults to $14 (20) on free-fire weapons.
 60  1 Current weapon power (number of times the weapon has been picked up?)
 61  1 Current weapon visage (Increases more slowly than 60, seems to be related to what the weapon's power level looks like when fired.)
 66  1 Selector for graphics in tiles $0F8-$0FF: 0=Illegal; 1=AREA;
       2=TIME; 3=GAME OVER; 4=BONUS; 5=PONYCA; 6-255=blank
 83  1 Frame count, in game time. (Updated by main loop, not VBLANK.)
 90  1 VRAM Blit Control. Bit 7 = VBLANK should copy target values to
       shadow registers.
 98  1 Target Y Scroll value.
 99  1 If nonzero, use the lower nametables as the base.
 9A  1 If nonzero, use the right-hand nametables as the base. Since
       Zanac is horizontally mirrored, this control byte has no visible
       effect.
 9B  1 Target X Scroll Value
 A9  1 Vertical Scroll subpixel count (256 subpixels = 1 pixel)
 AA  2 Current scroll speed, in subpixels/frame
 AC  2 Target scroll speed in normal play, in subpixels/frame
 AE  1 Actual number of full pixels scrolled this frame
 FC  1 Register shadow for Y Scroll value.
 FD  1 Register shadow for X Scroll value.
 FE  1 Register shadow for PPU_MASK.
 FF  1 Register shadow for PPU_CTRL. Other logic may alter the actual
       value of PPU_CTRL as needed by that logic, but it will preserve
       all other settings based on this and will restore to this value
       when it is done.
102  2 Magic Number. A fresh power-on is distinguished from a RESET by
       whether or not these two bytes are 35 53. If they are, the high
       score is not reset and the reset count at 19A is incremented.
       If they are not, first-time initialization is done (including
       setting these bytes to the right values).
17A  8 Score. Big-endian, one digit per byte, so a score of 12,300 is
       the bytes 00 00 00 01 02 03 00 00.
185  7 High Score. Works like 17A, but only 7 bytes long and the ones
       digit is assumed to be zero.
19A  1 Number of times you have soft-reset the console since power-on.
       This count is used to unlock Stage Select.
200 FF CPU-RAM copy of OAM data
460  1 Number of bytes of data ready to send to PPU.
461  2 PPU address for data, HIGH BYTE FIRST.
463 4D Data to copy to VRAM.

Register Shadows

In order to keep the display consistent at all times, PPU registers are *triple-buffered* in Zanac's engine. Scroll, mask, and control data are directly stored in the PPU's registers, and the values from $FC-$FF hold the values to reload them with each frame. On top of this, additional "target" values are held in $98-$9B which are updated quasi-atomically, assisted by bit 7 in $90.

Interesting Interactions

• The "super gun" does not actually exist on the main gun upgrade path. The super gun is bestowed by a routine at ROM offset $1973F which simply writes new hardcoded values to $35-$37 while leaving $33 untouched. This is why collecting additional power chips after getting the super gun downgrades you unless you had the three triple-shot gun (level 5) before gaining the super gun; the old value was still intact.
• When a Sart or Cargo enemy is destroyed, it will drop a Blue Lander pickup if $55 (the number of times the main gun has been fired) and $17e (thousands digit of your score) are equal to each other mod 64. This is why you always were urged to destroy the first Sart with your special weapon before doing anything else - zero equals zero, so it was the easiest way to get a free Lander.

Major open questions

• Where and how are the non-sprite parts of enemy/player objects held? There are a series of 26-element arrays from $0528 through at least $0715 that appear to describe object properties. This is a promising avenue of exploration.
• How do weapon selection and upgrades work? Zanac has some unusual corner cases in its mechanics where certain kinds of upgrades "lock" the player's upgrade status unless something dramatic happens.
• The VRAM data for transfer during VBLANK seems to have a number of possible formats, given the complexity of the function that does it. The characterization of 460-49F is almost certainly incomplete.
• Where and how is rank/System aggressiveness stored, what influences it, and what does it influence?