There's 2 machine code routines used in Grid Warrior, the 1st one
(at 30000) is used here because the 464 never had COPYCHR$(#0), so a small M/C Firmware patch could be written to read what's onscreen. Apart from being rather slow Vs. using traditional BASIC to read everything from an Array, compatibility issues slipped in when Amstrad updated to BASIC 1.1 because COPYCHR$(#0) was included and the Firmware updated, so reading Characters in different PENs or PAPER colours was a requirement in BASIC 1.1 and for Grid Warrior, this lead to an easier game on the 6128 as mentioned in The PC Mag+TAU edition of Grid Warrior. However, this was one of the earlier games I was able to patch up and have working the same on all Machines.
The 2nd M/C (at 32000) is perhaps more useful, which uses SCR HW ROLL to Roll the Screen up and then down when a Life is lost.