C11B: BD DB 1B   JSR $DB1B   Call random_number()
C11E: 81 40      CMPA #$40   A - 64?
C120: 24 F9      BCC $011B   Call random_number() while A > 63
C122: 97 2A      STA $2A     Store A in $202A

We're again generating a random number in the range 0---63, which suggests another placement into a sector of the galaxy map. Only a single position is generated, and the only thing that we haven't placed on the map is the player themselves; this is the initial sector into which they are spawned. We will in future refer to the data at $202A as the variable player_sector_number.

C124: 1F 89      TFR A,B     Copy A -> B
C126: 58         ASLB        B = B * 2
C127: 58         ASLB        B = B * 2
C128: 8E 25 19   LDX #$2519  Set X = $2519
C12B: 3A         ABX         X = X + B
C12C: 9F 36      STX $36     Store X in $2036
C12E: BD D3 79   JSR $D379   Jump to next initialization

Using the same sector number, we generate an offset into galaxy map memory, almost exactly as we did for the alien population in $C0B2-C0C0.

For example, suppose the player is randomly chosen to start in sector 10 ($0A). From this we create an address offset:

$$ \begin{array}{l@{\,}l} X &= \mathtt{$2519} + (10\times2\times2) \ &= \mathtt{$2541} \end{array} $$

But this time we store the address itself (e.g. $2541) at $2036. In other words, we are tracking the player's position in the galaxy, but it's not recorded in the galaxy map itself. Rather we can say we've created a variable which points to the relevant galaxy sector. Such a variable can in fact be called a pointer and we will refer to it with an asterix prefix as the *player_sector.

Deriving galaxy map properties

TODO: the derivation of properties fits here but the discussion of property syntax and addresses vs values does not. Note in our comments that we refer first to X as the player_location, but we then immediately refer to the value at that location as player_location.alien_count. This is because:

1. As a memory address (e.g. $2519), it tells us which sector of the galaxy the player is in
2. The value at that address (e.g. 19) is the number of aliens in that sector.

That's why we use the term player_location.alien_count; we're using the value at the player_location address, which is the number of aliens in the sector.

We can derive this and other properties of the sector by offsetting from the player_location address and comparing to what's shown on the sector map. TODO: diagram, overlay actual initialized map onto memory map to show correlation.

• player_location + 0 = player_location.alien_count
• player_location + 1 = player_location.station_health
• player_location + 2 = player_location.??? (initialized to a random number 0-255)
• player_location + 3 = player_location.station_type

Each sector uses only four bytes, so player_location + 4 puts us in the first byte of the next sector, so the value will be the alien count of the next sector.