Nice... I think it's looking very good.
I like the two-way disassembler. You may be able to write a complete IDE for Chip-8 and Super Chip from that.
Oh, as for adding, I think you can try doing some logical bit operations, too:
VF = ([VX] + value > 0xFF) ? 1 : 0; (sets carry flag)
VX = ([VX] + value) & 0xFF (this means keep the maximum as 0xFF)
I did a lot of logical bit operations in my interpreter.
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cChip interpreter WIP - current status: Release Candidate
LRx Filter RC - current performance rating: 9/10