Ch5 minor improvements (happi#141)

* Add bibliography ref to WAM

* Add links to code/modules

* Format: use monospace for code related parts

book.asciidoc

@@ -76,6 +76,8 @@ include::chapters/ap-beam_instructions.asciidoc[]
 
 include::chapters/ap-code_listings.asciidoc[]
 
+[[bibliography]]
+include::chapters/references.asciidoc[]
 
 //[index]
 //include::chapters/index.asciidoc[]

chapters/beam.asciidoc

@@ -24,9 +24,9 @@ components.
 
 === Working Memory: A stack machine, it is not
 
-As opposed to its predecessor Jam (Joe's Abstract Machine) which was a
+As opposed to its predecessor JAM (Joe's Abstract Machine) which was a
 stack machine, the BEAM is a register machine loosely based on WAM
-(TODO: cite). In a stack machine each operand to an instruction is
+ <<warren>>. In a stack machine each operand to an instruction is
 first pushed to the working stack, then the instruction pops its
 arguments and then it pushes the result on the stack.
 
@@ -49,8 +49,9 @@ add
 
 This code can be generated directly from the parse tree of
 the expression. By using Erlang expression and the modules
-+erl_scan+ and +erl_parse+ we can build the world's most simplistic
-compiler.
+https://erlang.org/doc/man/erl_scan.html[+erl_scan+] and
+https://erlang.org/doc/man/erl_parse.html[+erl_parse+] we
+can build the world's most simplistic compiler.
 
 [source,erlang]
 -------------------------------------------
@@ -177,16 +178,16 @@ Then we get the following code for the add function:
 ------------------------------------------
 
 Here we can see that the code (starting at label 2) first allocates a
-stack slot, to get space to save the argument _B_ over the function
-call _id(A)_. The value is then saved by the instruction
-_{move,{x,1},{y,0}}_ (read as move x1 to y0 or in imperative style: y0
-:= x1).
+stack slot, to get space to save the argument `B` over the function
+call `id(A)`. The value is then saved by the instruction
+`{move,{x,1},{y,0}}` (read as move `x1` to `y0` or in imperative style: `y0
+:= x1`).
 
 The id function (at label f4) is then called by 
-_{call,1,{f,4}}_. (We will come back to what the argument "1" stands for later.)
-Then the result of the call (now in X0)
-needs to be saved on the stack (Y0), but the argument _B_
-is saved in Y0 so the BEAM does a bit of shuffling:
+`{call,1,{f,4}}`. (We will come back to what the argument "1" stands for later.)
+Then the result of the call (now in `X0`)
+needs to be saved on the stack (`Y0`), but the argument `B`
+is saved in `Y0` so the BEAM does a bit of shuffling:
 
 Except for the x and y registers, there are a number of special
 purpose registers:
@@ -208,12 +209,12 @@ fields in the PCB.
     {move,{x,1},{y,0}}. % y0 := x1 (id(A))
 ------------------------------------------
 
-Now we have the second argument _B_ in x0 (the first 
-argument register) and we can call the _id_ function
-again _{call,1,{f,4}}_. 
+Now we have the second argument `B` in `x0` (the first
+argument register) and we can call the `id` function
+again `{call,1,{f,4}}`.
 
-After the call x0 contains _id(B)_ and y0 contains _id(A)_,
-now we can do the addition: _{gc_bif,'+',{f,0},1,[{y,0},{x,0}],{x,0}}_.
+After the call x0 contains `id(B)` and `y0` contains `id(A)`,
+now we can do the addition: `{gc_bif,'+',{f,0},1,[{y,0},{x,0}],{x,0}}`.
 (We will go into the details of BIF calls and GC later.)
 
 === Dispatch: Directly Threaded Code
@@ -297,7 +298,7 @@ int run(char *code) {
 You see, a virtual machine written in C does not need to 
 be very complicated. This machine is just a loop checking
 the byte code at each instruction by looking at the value
-pointed to by the _instruction pointer _ (ip).
+pointed to by the _instruction pointer_ (`ip`).
 
 For each byte code instruction it will switch on the instruction byte
 code and jump to the case which executes the instruction. This
@@ -424,7 +425,7 @@ values".
 We will look closer at the BEAM emulator later but we will take a
 quick look at how the add instruction is implemented. The code is
 somewhat hard to follow due to the heavy usage of macros. The
-STORE_ARITH_RESULT macro actually hides the dispatch function which
+`STORE_ARITH_RESULT` macro actually hides the dispatch function which
 looks something like: `I += 4; Goto(*I);`.
 
 [source, C]
@@ -496,10 +497,10 @@ in this code:
     {move,{x,1},{y,0}}.
 -------------------------------------------
 
-This code first saves the return value of the function call (x0) in a
-new register (x1). Then it moves the caller saves register (y0) to
-the first argument register (x0). Finally it moves the saved value in
-x1 to the caller save register (y0) so that it will survive the next
+This code first saves the return value of the function call (`x0`) in a
+new register (`x1`). Then it moves the caller saves register (`y0`) to
+the first argument register (`x0`). Finally it moves the saved value in
+x1 to the caller save register (`y0`) so that it will survive the next
 function call.
 
 Imagine that we would implement three instruction in BEAM called
@@ -633,7 +634,8 @@ need to do explicit memory management. On the BEAM level, though, the
 code is responsible for checking for stack and heap overrun, and for
 allocating enough space on the stack and the heap.
 
-The BEAM instruction +test_heap+ will ensure that there is as much
+The BEAM instruction https://github.com/erlang/otp/blob/OTP-23.0/lib/compiler/src/genop.tab#L118[+test_heap+]
+will ensure that there is as much
 space on the heap as requested. If needed the instruction will call
 the garbage collector to reclaim space on the heap. The garbage
 collector in turn will call the lower levels of the memory subsystem