mal-note/operations.md

Operations in MAL

In MAL there is one main type of operation, and several sub-operations which can be attached.

ALU Operations

There are three parts to any ALU operation...

Choosing which registers to write to

You can write to any number of registers, as long as those registers can be written to. [./registers.md](See here for the distinctions between registers.)

The formatting for is, is simply to add any number of register names, and a equals sign afterwards:

REG1 = ... = REGN =

Choosing what to set them to

This is the fun part. There are 64 possible ALU operations, and is increased to 192 distinct operations when working with shifter, but a large number of them produce nonsense, and thus ain't available from MAL.

Assuming A is any readable register, except H, the following is the full list of operations before shifting:

A
-A
INV(A)
A + 1

H
INV(H)
H + 1

A AND H
A OR H
A + H
A + H + 1
H - A
H - 1

0
1
-1

It is important to notice how not all operations are equally valid for both A and H, for example -A have no H counterpart.

Possibly do some shifting

The shifter is hooked directly up to the ALU output, which means that shifting always happens last. Shifting is added to the operations, after any other operation. The MIC1 only support left shifting 8, and right rotating 1:

<< 8
>> 1

Summary

It is thus possible to take several different smaller parts, and combine them into a large (though useless) operation like this:

MDR = H = MAR AND H << 8

Other Operations

In addition to the ALU operations, some misc other operations exist. Most of these can be executed in the same cycle as an ALU operation.

Adding one or more of these to an ALU operation, can be done by separating them with a semicolon, like so:

MDR = A + 1; wr; fetch; goto(MBR)

Lets go over them quickly:

• rd: Reads the MARth word from memory to MDR. MAR can be set same clock cycle.

• wr: Writes MDR to the MARth word in memory. MAR and MDR can be set same clock cycle.

• fetch: Reads the PCth byte from program memory to MBR. PC can be set same clock cycle.

• goto LABEL1: Specifies a specific label to jump to. Equal to setting the BPC to the position of the label. Usable any clock cycle.

• if (X) goto LABEL1; else goto LABEL2: Jumps to either label depending upon the value of X. X can be the virtual registers Z or N. Usable any clock cycle.

• goto (MBR): Sets the BPC to the value of MBR. MBR can be loaded same clock cycle.

• goto (MBR or 0x100): Sets the BPC to the value of MBR OR 0x100. Adding by 0x100, but faster. MBR can be loaded same clock cycle.

Using the last two might be tricky.