The best place to detect if the game has loaded is a combination of the !MSYNC (Master Sync pin 13), and the SR1 (pin 9) of the STIC (U4).

In short, the STIC comes out of reset when its !RESET (pin 14) is high. This is directly tied to the reset button, so unless that is pressed, the STIC should boot up.

When it starts, it attempts to load the game from the game slot using the BC1, BC2, BDIR, SR2, SDx, MSYNC and a few other pins.

If there is no game inserted, then SR1 (pin 9) is long duty cycle pulses.

If there is a game inserted, it should pulse short bursts.

With that, check the !MSYNC (pin 13). If the game has successfully loaded this pin will stay high permanently.

If the game failed to load, the !MSYNC will be constantly cycled, and the game load attempted over and over.

The actual game load takes place via the CPU (U1), System RAM (U2), the 9504 Exec (U3) and the game cartridge.

The CPU

As part of the Exec chips (U3 / U9) functions, they also use Static RAM (U7 and U8) and Graphics ROM (U5).

Graphics ROM and the Graphics RAM store shapes of images used on-screen. GRAM is programmable and GROM is not. GRAM has space for 64 program-defined 8x8 bit images. Both background and moving object images are stored here. GROM contains 213 predefined 8x8 bit image locations.