From e22bff1289ce0c0495243966c4a60f6fcde8c588 Mon Sep 17 00:00:00 2001
From: Jon Michael Aanes <jonjmaa@gmail.com>
Date: Wed, 16 Dec 2015 23:26:04 +0100
Subject: [PATCH] Added README / Base level. Added note on operations.

---
 README.md     |  10 +++++
 operations.md | 101 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 111 insertions(+)
 create mode 100644 README.md
 create mode 100644 operations.md

diff --git a/README.md b/README.md
new file mode 100644
index 0000000..978c72c
--- /dev/null
+++ b/README.md
@@ -0,0 +1,10 @@
+MAL Note
+========
+
+A collection of notes relating to the MIC1 MAL language, described in the book
+**Structured Computer Organization** by **Andrew S. Tanenbaum**.
+
+Pages:
+
+-	[./registers.md](Registers)
+-	[./operations.md](Operations)
diff --git a/operations.md b/operations.md
new file mode 100644
index 0000000..2059320
--- /dev/null
+++ b/operations.md
@@ -0,0 +1,101 @@
+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 `MAR`th word from memory to `MDR`. `MAR` can be set same
+	clock cycle.
+-	`wr`: Writes `MDR` to the `MAR`th word in memory. `MAR` and `MDR` can be set
+	same clock cycle.
+-	`fetch`: Reads the `PC`th 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.