bits insn notes
0000 1000 LD 16m16,SP loads SP
-0000 1001 ADD HL, BC
0001 0000 STOP
-0001 1001 ADD HL, DE
-0010 1001 ADD HL, HL
-0011 1001 ADD HL, SP
0111 0110 HALT Danger! Helvetica!
1100 1011 - - - see two-byte opcodes below
-1110 1000 ADD SP, imm8
-1111 1000 LDHL SP, imm8 load SP+n (signed n) into HL
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