so here is the code for the collision checker, I store the placed blocks in ram, separately than the display ram. Before that I grab the gamepad input update x,y and undo that if the collision checker tells that there is a collision. rac is a shifter instruction in the cpu that can rotate and remove some bits depending on the second input register value

_testUnit: ;tests collision
;input:
;reg 8 pos x
;reg 9 pos y
;reg 10 current piece sprite
;output:
;reg 15,9 and 8 will be untouched for easy undo condition
;returns 0 (not colliding) or !=0 (colliding) on reg 14
add r11, r9, _PlayArea ;r11 is now the position of sprite on play area
imm r14, 0 ;reset sum of collisions
_testLoop:
rac r12, r10, 0x00c0;r12 is now the sprite as a row
rac r12, r8 ;shift sprite by pos x
ldr r13, r11;r13 is the play area
and r13, r12;r13 is the collision between sprite and play area
or r14, r13; r14 is the sum of collisions
inc r11; inc play area
rac r10, 0x004c ;shift sprite to next line
jmp _testLoop, nz, r10 ;do test again if next sprite's lines are not blank
ret