Editing Microcode (section)

===Horizontal microcode===
"Horizontal microcode has several discrete micro-operations that are combined in a single microinstruction for simultaneous operation."<ref name=Kent2813 /> Horizontal microcode is typically contained in a fairly wide control store; it is not uncommon for each word to be 108 bits or more. On each tick of a sequencer clock a microcode word is read, decoded, and used to control the functional elements that make up the CPU.

In a typical implementation a horizontal microprogram word comprises fairly tightly defined groups of bits. For example, one simple arrangement might be:
{| class="wikitable"
|-
| Register source A || Register source B || Destination register || [[Arithmetic and logic unit]] operation || Type of jump || Jump address
|}

For this type of micromachine to implement a JUMP instruction with the address following the opcode, the microcode might require two clock ticks. The engineer designing it would write microassembler source code looking something like this:
{{sxhl|
   # Any line starting with a number-sign is a comment
   # This is just a label, the ordinary way assemblers symbolically represent a 
   # memory address.
 InstructionJUMP:
       # To prepare for the next instruction, the instruction-decode microcode has already
       # moved the program counter to the memory address register. This instruction fetches
       # the target address of the jump instruction from the memory word following the
       # jump opcode, by copying from the memory data register to the memory address register.
       # This gives the memory system two clock ticks to fetch the next 
       # instruction to the memory data register for use by the instruction decode.
       # The sequencer instruction "next" means just add 1 to the control word address.
    MDR, NONE, MAR, COPY, NEXT, NONE
       # This places the address of the next instruction into the PC.
       # This gives the memory system a clock tick to finish the fetch started on the
       # previous microinstruction.
       # The sequencer instruction is to jump to the start of the instruction decode.
    MAR, 1, PC, ADD, JMP, InstructionDecode
       # The instruction decode is not shown, because it is usually a mess, very particular
       # to the exact processor being emulated. Even this example is simplified.
       # Many CPUs have several ways to calculate the address, rather than just fetching
       # it from the word following the op-code. Therefore, rather than just one
       # jump instruction, those CPUs have a family of related jump instructions.
|ucode}}
For each tick it is common to find that only some portions of the CPU are used, with the remaining groups of bits in the microinstruction being no-ops. With careful design of hardware and microcode, this property can be exploited to parallelise operations that use different areas of the CPU; for example, in the case above, the ALU is not required during the first tick, so it could potentially be used to complete an earlier arithmetic instruction.