Micro Instructions

Microinstructions are the hardware-level of what instructions actually are. They are singular values, which can be either true or false. In hardware, this would simply be a wire turning something on or off.

This is necessary in order to break-down instructions into their base parts, because they cannot be executed all at once, since instructions often require multiple steps, even using the same part of the CPU.

Many of the microinstructions are placed in groups. This is done to reduce the amount of index space they require in binary form. For example, there are 7 different Read Microinstructions. But since all of them write a value to the bus, we can obviously only use one at a time, unless we want to mess up our data. So instead of having a single bit dedicated to every single one to tell our CPU which to turn on or off, we can just group them together and index them.
In the sample below, this means what would be 1111111 can be reduced to 111, saving 4 bits for other instructions.

ALU Microinstructions:

// 000000000000 11  compact
SU : enable subtraction in ALU
MU : enable multiplication in ALU
DI : enable division in ALU

Read Microinstructions:

// 000000000 111 00  compact
RA : read from reg A to bus
RB : read from reg B to bus
RC : read from reg C to bus
RM : read from memory to bus at the address in mem addr. register
IR : read from lowest 12 bits of instruction register to bus
CR : read value from counter to bus
RE : read from expansion port to bus

Write Microinstructions:

// 00000 1111 00000  compact
WA : write from bus to reg A
WB : write from bus to reg B
WC : write from bus to reg C
IW : write from bus to instruction register
DW : write from bus to display register
WM : write from bus to memory
J  : write from bus to counter current value
AW : write lowest 12 bits from bus to mem. addr. register
WE : write from bus to expansion port

Uncategorized Microinstructions:

// 11111 000000000  full
FL : update flags register
EI : end instruction, resets step counter to move to next instruction
ST : stop the clock
CE : enable incrementing of counter
EO : read from ALU to bus