finished discussion of newly introduced instructions

section_2/bitfields/README.md

@@ -12,8 +12,7 @@ Bit bashing is the manipulation of individual bits. Bit
 bashing goes to the very core of the C language. Remember that C is a
 high level assembly language, as we argue in Section 1 of this book.
 And C is the (later) language in which Unix was implemented and indeed,
-C was
+C was developed specifically to implement Unix.
-developed specifically to implement Unix. 
 
 Since an operating system directly
 interfaces with hardware - the C language grew to have features

section_2/bitfields/review.md

@@ -2,15 +2,19 @@
 
 ## Overview
 
-Our discussion of implementing ourselves what the C / C++ compiler gives us
+Our discussion of implementing ourselves what the C / C++ compiler gives
-led us to use six new instructions. This chapter reviews those instructions.
+us led us to use six new instructions. This chapter reviews those
+instructions. In addition to describing what an instruction does, we
+will try to indicate what the instruction is "good for."
 
 ## `and`
 
-The `and` instruction is pretty much what you would expect. It implements
+The `and` instruction is pretty much what you would expect. It
-the `&` operator from C and C++. That is, the bitwise and operator.
+implements the `&` operator from C and C++. That is, the bitwise and
+operator.
 
-The `and` instruction comes in a number of flavors
+The `and` instruction comes in a number of flavors including for
+use with immediate values.
 
 ### `and` Immediate
 
@@ -27,20 +31,19 @@ There are limits to the bit width of `imm` because it has to fit within
 the `and` instruction. If you exceed the allowable width of `imm`, the
 assembler will be glad to insult you.
 
-It is possible that a `mov`
+It is possible that a `mov` instruction will allow your immediate value.
-instruction will allow your immediate value. You'd follow up with an
+You'd follow up with an `and` using a register than an immediate value.
-`and` using a register than an immediate value.
 
-If your immediate value is too large for a `mov` then put the value
+If your immediate value is too large for a `mov` then put the value in
-in RAM and `ldr` it into a register and proceed.
+RAM and `ldr` it into a register and proceed.
 
 We would love to tell you what the rules are for an immediate value in
 the `and` instruction, but they are not obvious and in fact are very
-complex. Our advice, try the immediate value you have in mind and if
+complex. Our advice, try the immediate value you have in mind and if it
-it works, great. Otherwise, see above.
+works, great. Otherwise, see above.
 
-A slight variation on this `and` instruction uses a register where
+A slight variation on this `and` instruction uses a register where the
-the preceding one uses an immediate value.
+preceding one uses an immediate value.
 
 ```asm
     and    rd, rs, rm
@@ -65,18 +68,29 @@ These mean:
 | `asr` | arithmetic shift right | shifts right introducing duplicates of the previous most significant bit |
 | `ror` | rotate right | shifts right introducing the bits shifted out back in from the left |
 
-There are other two similar `and` instructions. These are the immediate and
+There are other two similar `and` instructions. These are the immediate
-the register versions of `ands`. `ands` is the same as `and` with the addition
+and the register versions of `ands`. `ands` is the same as `and` with
-that the CPU's condition bits are updated by the instruction permitting a
+the addition that the CPU's condition bits are updated by the
-conditional branch to follow the instruction.
+instruction permitting a conditional branch to follow the instruction.
+
+*`and` is the basis of bit bashing and is used to clear bits.
+Put `and` together  with the or instructions and a couple of other basic
+logic instructions, out pops a computer.*
+
+A shorthand way of clearing specific bits is the `bic` instruction.
+Use of `bic` can avoid having to negate the bits in a mask prior to
+and'ing.
 
 ## `bfi`
 
-This instruction mnemonic stands for Bit Field Insert. The word *Insert* should
+This instruction mnemonic stands for Bit Field Insert. The word *Insert*
-really be copy. The official ARM [documentation](https://developer.arm.com/documentation/dui0801/g/A64-General-Instructions/BFI) explains this instruction
+should really be copy. The official ARM
-very well so we'll repeat it here:
+[documentation](https://developer.arm.com/documentation/dui0801/g/A64-General-Instructions/BFI)
+explains this instruction very well so we'll repeat it here:
 
-*Bit Field Insert copies any number of low-order bits from a source register into the same number of adjacent bits at any position in the destination register, leaving other bits unchanged.*
+*Bit Field Insert copies any number of low-order bits from a source
+register into the same number of adjacent bits at any position in the
+destination register, leaving other bits unchanged.*
 
 The instruction has the following format:
 
@@ -84,16 +98,29 @@ The instruction has the following format:
     bfi    rd, rs, lsb, width
 ```
 
-Starting at bit 0 of `rs`, `width` bits are copied to `rd` starting at bit
+Starting at bit 0 of `rs`, `width` bits are copied to `rd` starting at
-`lsb`.
+bit `lsb`.
 
-The `bfi` instruction replaces as many as three instructions: likely a shift,
+The `bfi` instruction replaces as many as three instructions: likely a
-an `and` and an `orr`.
+shift, an `and` and an `orr`.
+
+*The preceding has described what `bfi` does but what is it for? Suppose
+you have a multiple bit field in a device register. Using `bfi` makes
+it easy to copy the right number of bits from a source directly into
+the bits in the middle of the destination without need of other
+bit-bashing.*
+
+The *opposite* of `bfi` is `bfx`.
+
+`ubfiz` first zeros the destination register then copies in the
+specified bits from the source. The `u` means don't consider the
+sign bit as special. The `z` is what causes the zero fill first and
+sets it apart from `bfi`.
 
 ## `mvn`
 
-This instruction takes only takes two operands (but permits an optional shift
+This instruction takes only takes two operands (but permits an optional
-to be explained below).
+shift to be explained below).
 
 The basic syntax is:
 
@@ -101,7 +128,8 @@ The basic syntax is:
     mvn    rd, rs
 ```
 
-This flips all the bits in the source and copies them to the destination.
+This flips all the bits in the source and copies them to the
+destination.
 
 In addition to the basic instruction, there is also:
 
@@ -109,11 +137,41 @@ In addition to the basic instruction, there is also:
     mvn    rd, rs, *shift* num_bits
 ```
 
-The *shift* and `num_bits` function the same way as described above including
+The *shift* and `num_bits` function the same way as described above
-the option to use `*shift*` as one of `lsl`, `lsr`, `asr` or `ror`.
+including the option to use `*shift*` as one of `lsl`, `lsr`, `asr` or
+`ror`.
+
+*Note the `mvn` is different from `neg` in that `mvn` is a bitwise
+operation while `neg` is aware of what makes a negative integer value.
+That is, if your goal is to turn a positive integer into a negative
+integer, use `neg`. If your goal is to negate bits for bit bashing
+purposes, use `mvn`.*
 
 ## `lsl`
 
+**Logical Shift Left** moves bits to the left shifting 0 into any
+vacated positions.
+
+```asm
+	lsl		rd, rs, *number of bits*
+```
+
+*`lsl` is most often used for bit bashing as well as for a fast
+multiplication by a power of 2.*
+
+The instruction `asl` is a synonym for `lsl`. The `a` is `asl` means
+"arithmetic". There is no difference between a logical and
+an arithmetic shift in the **leftward** direction. There is, however,
+a difference in the **rightward** direction in that `asr` replicates
+the sign bit where `lsr` still moves in zeros in any bit positions
+vacated.
+
 ## `orr`
 
-## `ubfiz`
+This is the plain vanilla *or* instruction. It is used extensively
+for bit bashing as it is how bits can be set to 1.
+
+```asm
+	orr		rd, rs, rm  # rm is another register
+	orr		rd, rs, imm
+```