What is the primary book used

There isn't one. Most people don't learn this stuff by reading a book.

The best way to learn is by looking at actual assembly code, then research what each instruction does. I wouldn't start with actual compiler generated code. Being computer generated it's often quite messy and obviously undocumented. Best to start with easier to read code like the example I've included below — a simple "print Hello World" in CISC, then in RISC.

Notice CISC uses mov, int and xor, while RISC uses mov, ldr, and svc. You should look those up in a manual (plenty of free ones online) but in simple terms:

• mov: move memory from one place to another. RISC and CISC have the same instruction but but they're not identical
• int: means interrupt, essentially stop execution (for a moment) and hand execution over to other software
• xor: modifies a value (an XOR operation)
• ldr: is "load register", it loads a value from elsewhere in memory
• svc: means "supervisor call" which, is used in much the same way as int. The code is asking the kernel to do something (once to write to stdout, and once to terminate the program).

section .data
    helloWorld db 'Hello World',0xa  ; 'Hello World' string followed by a newline character

section .text
    global _start

_start:
    ; write(1, helloWorld, 13)
    mov eax, 4          ; system call number for sys_write
    mov ebx, 1          ; file descriptor 1 is stdout
    mov ecx, helloWorld ; pointer to the string to print
    mov edx, 13         ; length of the string to print
    int 0x80            ; call kernel

    ; exit(0)
    mov eax, 1          ; system call number for sys_exit
    xor ebx, ebx        ; exit status 0
    int 0x80            ; call kernel

.section .data
helloWorld:
    .asciz "Hello World\n"

.section .text
.global _start

_start:
    ; write(1, helloWorld, 13)
    mov r0, #1                  ; file descriptor 1 is stdout
    ldr r1, =helloWorld         ; pointer to the string to print
    mov r2, #13                 ; length of the string to print
    mov r7, #4                  ; system call number for sys_write
    svc 0                       ; make system call

    ; exit(0)
    mov r0, #0                  ; exit status 0
    mov r7, #1                  ; system call number for sys_exit
    svc 0                       ; make system call

You probably want to look for books on reverse engineering. And a book on assembly for your CPU.

I learned assembly language for VAX-11 (this was like 30+ years ago) in a CS class. We also learned 6502 assembly in a computer engineering class. Neither book would help you. You want a book specific to whatever CPU you're using.

Now, I never took it, but friends in college took a CS Compilers course where they learned the basics of writing a compiler. But that's not what you're talking about though it might help.

Trying to understand what a program does is reverse engineering. And a tool like IDA Pro would help you understand subroutines, variables, flow, library calls, and so on.

A debugger will be invaluable for seeing a program execute one instruction at a time.

You would need to know the assembly language for your CPU. And it would help to become familiar with certain patterns. I haven't done much assembly (but I have done assembly on a few different CPUs) nor much reverse engineering so I'm not sure I can lend a whole lot of insight there.

As you learn assembly instructions, you will start to understand how loops, subroutines, if/then/else, and other things are accomplished for your CPU.

For example, if/then/else and loops are often accomplished with conditional branching. The conditions are based on CPU flags (bits in the Status Register) that are set by a comparison instruction. You'll start to recognize how if/then/else and loops and other things are commonly implemented in assembly (without necessarily having to study the compiler; it will be obvious without knowing anything but assembly).

Another example might be how C structs are implemented. Some CPUs provide convenient memory addressing modes for structs, some don't. Nearly all I am familiar with provide a convenient way to reference arrays with a simple index.

Subroutines are jumps to a set location and at the end of that code is a return instruction. Usually registers have to be saved when jumping and restored when returning. Arguments to the subroutines are pushed on to the stack either by value or by reference. Return value is provided through some convention (machines with lots of registers might always use one particular one for return).

I guess bottom line, learn assembly for your particular CPU, then take a crack at using a debugger and disassembler / reverse engineering tool.

I'm not entirely sure I follow why that is needed to learn how to do branching in forth but I only vaguely remember that language. Maybe if I did it would be more clear.

Anyway I hope this helps at least a little.

I don't understand...

Are you trying to learn how to make compilers/interpreters? Are you trying to learn how to write assembly like the compiler does? Or are you trying to have a deeper understanding of those languages?

Those are all very different things. (Computer architecture is also one very different thing, but from what I understand that's not what you want.)

I learned through three things:

1. writing some basic functions in assembly code by hand for a course (not many)
2. implementing a basic compiler back-end in llvm (any similar IR or assembly target would do)
3. learning the principles other people were using to write fast code (in my case game engine developers)

The first two things helped me understand how common code constructs are translated to assembly, so I can do a rough projection in my head when skimming a C function. Nowadays you can get quite far just by playing around on godbolt.

The third thing helps surface the less visible aspects of CPUs. After learning how a few low-level optimisations work, all the principles and explanations start to repeat, and 90% of them apply to every modern architecture. You can set out with specific high-level questions, like:

• why is iteration faster with an array than a linked list?
• what does vectorisation mean?
• what is a "struct of array" optimisation?
• why does the ECS pattern make game engines fast?

Very quickly you'll find lots of insightful articles and comments explaining things like CPU caching, prefetching, branch prediction, pipelining, etc.

I have no book recommendations for you. I've found all the best information is freely online in blogs and comment sections, and that the best way to direct my learning is to have a project (or get employed to do low-level stuff). Might be different for you though!

For reverse engineering you probably have to study some assembly output of compilers since the methods of implementing stuff like C++ vtables can be a bit intricate. There are also some books you can read. This bundle has one that I haven't looked at:

https://www.fanatical.com/en/bundle/effective-cybersecurity-prevention-bundle

There are decompilation tools that can recognize some of that stuff automatically too (idk if Ghidra does that).

Handwritten asm code will generally not look like compiled code. The Forth interpreters you are looking at are probably a good place to start.