Simplify isROM
Make do with just 16K RAM
Cleanup
Fix video RAM size
Implement CLC and ADC_ZP
Implement LDY_Abs_X
WriteIndirect: allow transformation of target address.
This enables implementing STA_Ind_Y &c.
Fill the screen in a loop, and change the origin address to $F000
TXA, TYA: update flags
BNE, BEQ: offset is signed
Fix opcode for STX_ZP
Fix opcode for ADC_ZP
Implement LDA_Abs_X and ADC_IMM
Simplify ADC_ZP
Fix STX to store X, not A...