Editing Orthogonal instruction set (section)

===The 8080 and follow on designs===
The 8-bit [[Intel 8080]] (as well as the 8085 and 8051) microprocessor was basically a slightly extended accumulator-based design and therefore not orthogonal. An assembly-language programmer or compiler writer had to be mindful of which operations were possible on each register: Most 8-bit operations could be performed only on the 8-bit [[Accumulator (computing)|accumulator]] (the A-register), while 16-bit operations could be performed only on the 16-bit pointer/accumulator (the HL-register pair), whereas simple operations, such as increment, were possible on all seven 8-bit registers. This was largely due to a desire to keep all opcodes one byte long.

The [[binary-compatible]] [[Z80]] later added prefix-codes to escape from this 1-byte limit  and allow for a more powerful instruction set. The same basic idea was employed for the [[Intel 8086]], although, to allow for more radical extensions, ''binary''-compatibility with the 8080 was not attempted here. It maintained some degree of non-orthogonality for the sake of high code density at the time. The 32-bit extension of this architecture that was introduced with the [[80386]], was somewhat more orthogonal despite keeping all the 8086 instructions and their extended counterparts. However, the ''encoding-strategy'' used still shows many traces from the 8008 and 8080 (and Z80). For instance, single-byte encodings remain for certain frequent operations such as [[Stack (abstract data type)|push and pop]] of registers and constants; and the primary accumulator, the [[EAX register]], employs shorter encodings than the other registers on certain types of operations. Observations like this are sometimes exploited for [[code optimization]] in both compilers and hand written code.