The formats of the 32-bit instructions for this architecture are shown in the diagram below.
The formats of the 32-bit instructions are shown here:
The instructions shown in lines 41, 42, and 43 of the diagram, separated from the other instructions by a horizontal line, are only available in those block formats which specify that some instruction slots are not to be decoded.
In blocks with headers in the fifth, sixth, and seventh formats, all the basic memory-reference instructions in lines 1 through 19 of the diagram above, both the unaligned ones shown in lines 1 through 5 and the aligned ones shown in lines 6 to 22, are replaced by basic memory-reference instructions in the format shown below:
Clearly, it makes more sense to use one large contiguous area of opcode space for a format of memory-reference instructions without limitations than to divide the area up into several different formats of memory-reference instructions, each with different limitations, as a way of reducing the impact of limitations in the memory-reference instructions.