Table of number of aliens left in each row. Starts of with 5 rows of 11 aliens. The top screen row is first byte.

Table contains number of aliens in a column. There are 11 aliens per row, so that's 11 columns. When an alien is hit its row value in this table is reduced by one. When the column gets to zero that means there are no aliens left in that column. This is used to manage the horde as they move right to left and back again.

Band value used by several of the alien band handling functions. It gets passed around in a rather messy way. During alien movement code it's the band which is being moved. 1 is the top row, 5 is the bottom row.

This is set during the period when aliens are moving downwards (as opposed to sideways)

This stores the lowest screen row (0 to 4, where 4 is the bottom alien row) which has at least one alien in it. So if you clear the bottom row this will go from 4 to 3. This is used to cycle up through up 5 rows of aliens looking to see if the player's bullet is in that row or whether an alien bullet can start in that row.

This is the direction the horde is moving. 1 means left to right. -1 (255) means right to left.

Which column, starting from the left, still has aliens in it. Starts off as 0, goes to 1 when left most column of aliens is cleared, etc. It reflects the gap to the left side of the horde.

Which column, starting from the right, still has aliens in it. Starts off as 11, goes to 10 when right most column of aliens is cleared, etc. It reflects the gap to the right side of the horde.

Bullet data, this one is xpos,ypos in pixels

x,y pairs for up to 4 alien bullet screen locations. The y value is set 0 when the entry in the array isn't in use.

Spaceship direction. 0 means no spaceship in play. 1 means one travelling left to right, -1 means one travelling right to left