+* The Compressed Major Opcode is in bits 5-7.
+* Minor opcode in bit 8.
+* In some cases bit 9 is taken as an additional sub-opcode, followed
+ by bits 0-4 (for CR operations)
+* M+N mode-switching is not available for C-Major 0b001 or 0b111
+* 10 bit mode may be expanded by 16 bit mode, adding capabilities
+ that do not fit in the extreme limited space.
+
+### Immediate Opcodes
+
+only available in 16-bit mode, and only available when M=1 and N=1
+
+ | 0 | 1 | 2 3 4 | | 567.8 | 9ab | c d e | f |
+ | 1 | o2 | RT | | 010.0 | RB|0 | offs | 1 | addi.
+ | 1 | o2 | RT | | 010.1 | RB|0 | offs | 1 | addis.
+ | 1 | o2 | | 011.0 | RB | offs | 1 | cmpdi
+ | 1 | o2 | | 011.1 | RB | offs | 1 | cmpwi
+ | 1 | o2 | | 100.0 | RT | offs | 1 | sti
+ | 1 | o2 | | 100.1 | RT | offs | 1 | fstwi
+ | 1 | o2 | | 101.0 | RA | offs | 1 | ldi
+ | 1 | o2 | | 101.1 | RA | offs | 1 | lwi
+ | 1 | o2 | | 110.0 | RA | offs | 1 | flwi
+ | 1 | o2 | | 110.1 | RA | offs | 1 | fldi
+
+* Note that bc is included (below)
+* immediate is constructed from offs (LSBs) and o2 (MSB)
+* for LD/ST, offset is aligned. 8-byte: o2||offs||0b000 4-byte: 0b00
+* SV Prefix over-rides help provide alternative bitwidths for LD/ST
+* RB|0 if RB is zero, addi. becomes "li" (this only works if RT takes
+ part of opcode).
+