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Update description of IR in README

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@ -231,13 +231,13 @@ Branch to address of label if [**O-flag**](#o) is **0**.
### BOP
Branch to address specified by entry in optable pointed to by highest **4** bits
Branch to address specified by entry in optable pointed to by highest [**OP**](#op)
in [**IR**](#ir).
### BAM
Branch to address specified by entry in addressing mode pointed to by M-bits in
[**IR**](#ir).
Branch to address specified by entry in addressing mode pointed to by [**M**](#m)
in [**IR**](#ir).
### BST
@ -366,7 +366,7 @@ and write operations.
### AR
The accumulator register. This register can only be written to as the result of
an ALU operation. This is to say, that AR is indirectly writable via the ALU,
an ALU operation. This is to say, that **AR** is indirectly writable via the ALU,
but is nonetheless directly readable via the bus.
### PM
@ -385,24 +385,34 @@ storing ephemeral or intermediate values during a computation.
### IR
The instruction register. This register offers extra functionality such as
**K1**- and **K2**-table addressing via the **OP** and **M** bits respectively.
The **GRx** and **M** bits can also be used to address a specific general
register via the **GR** multiplexer (see [**GR**](#gr)).
**K1**- and **K2**-table addressing via the [**OP**](#op) and [**M**](#m)
bits respectively. The [**GRx**](#grx) and [**M**](#m) bits can also be used
to address a specific general register via the [**GR**](#gr) multiplexer.
The bit-level layout of IR (from MSB to LSB) is as follows:
Bit-level layout of IR (from MSB to LSB, left-to-right):
* **OP** : 4 bits (machine instruction). Can be used to jump to a
microinstruction address given by **K1** at the index specified by **OP**
| **OP** | **GRx** | **M** | **ADR** |
|:------:|:-------:|:------:|:-------:|
| 4 bits | 2 bits | 2 bits | 8 bits |
* **GRx**: 2 bits (GR multiplexer selector)
* **M** : 2 bits (Addressing mode)
##### OP
Machine instruction. Indexes **K1**-table.
##### GRx
[**GR**](#gr) multiplexer selector.
##### M
Addressing mode. Indexes **K2**-table.
##### ADR
[**ASR**](#asr-1) address argument.
* **ADR**: 8 bits (ASR address argument)
### GR
This is a shorthand for accessing the general register currently made available
by the GR multiplexer when said MUX is controlled by the **GRx** bits in
by the GR multiplexer when said MUX is controlled by the [**GRx**](#grx) bits in
[**IR**](#ir).
**NOTE**: Only one GR can be accessed per cycle. Which register this is (of
@ -410,7 +420,7 @@ the four available registers) is determined by the value in [**IR**](#ir).
### GRM
This is a shorthand for accessing the general register currently made available
by the GR multiplexer when said MUX is controlled by the **M** bits in
by the GR multiplexer when said MUX is controlled by the [**M**](#m) bits in
[**IR**](#ir).
**NOTE**: Only one GR can be accessed per cycle. Which register this is (of