Writing Hello world from scratch Part I

Making a new ISA

“If you wish to make an apple pie from scratch, you must first invent the universe.”

― Carl Sagan

What is an ISA?

An instruction set architecture, or ISA, describes the instruction set of a processor and its behavior. This is not enough to know how a processor works but this is the information needed to operate it.

I made my own ISA named Reflet which you can check out on the Reflet GitHub repository.

Designing a new ISA

Wishlist

Why would one design a new ISA? With RISC-V and MIPS being open ISA with great support, there is not much point in doing so. Personally, I mostly did this for fun and artistic aspiration. I had a few ideas trotting in my head about what a fun ISA could be. I did want wanted to make a simple ISA, but rather a family of ISA for 8-bit, 16-bit, 32-bit, 64-bit, or even more bit-having processors that all share the same instruction set. At first, I even wanted to be able to make a processor with exotic word sizes but I scrapped that idea. I also wanted a Von Newman architecture and having instructions being a single indexable amount of memory. Those three needs combined meant that I needed to have instruction on only 8 bits. This is quite low, even some 8-bit computers use 16 bits instructions.

On the other hand, I really liked the idea of instructions being 8 bits wide. This meant that I would probably end up with every possible byte being a valid instruction (and this ended up being the case). I find it quite beautiful that any binary file would be a valid program in my instruction set.

Another thing I wanted is to have a lot of general-purpose registers. I really like the idea of running a processor only from its ROM and registers, without using any RAM, like the HP nanoprocessor. This need does not mix well with having 8-bits wide instruction as indexing this register takes space.

Solutions

To solve those issues, I needed to have an accumulator-like register. Indeed, had I used only general-purpose registers, when using two registers in each instruction, I would have needed to use less than 3 bits to access each register. This would have meant having less than 8 general-purpose registers. I could have had different categories of registers that are addressed differently but I really did not like this idea.

The solution is to have an accumulator-like register that is implicitly used by (almost) all instruction. Some instructions have a 4-bit opcode and a 4-bit operand that let me index 16 registers. Those instructions use both the working register (name of my accumulator-like register) and another register. Some other instructions only have an 8-bit opcode and don't interact with other registers than the working register. In the end, I ended up with the following instruction set:

One could notice that if I needed to squeeze a bit more instruction, I could. For example, I could replace the slp instruction with read WR or cpy WR which are two instructions that do not change the state of the processor. Furthermore, I could fuse the debug, quit, and maybe some other instructions with a single instruction that uses the state of the working register to control its behavior. I might do that in the future if I feel like the instruction set needs more instructions.

As far as registers go, I have the working registers, a status register, a stack pointer, and a program counter as special registers. This left 12 general-purpose registers which is quite handy. As far as the status register go, as I want the behavior to be very similar with all word size, only the 8 LSB are used for status.

The complete (but still very small) documentation is available on the Reflet GitHub repository.

Simulator

Before writing the hardware description for my processor, I made a simulator. The point of the simulator is to test easily if programs I compiled to Reflet machine code work.

The simulator can only do very simple I/O, reading chars from stdin one at a time and writing chars to stdout one at a time. Fortunately, this is just enough to write a Hello, world! program. Outputting character works by writing the desired character to at the address 0x1 and then, writing 0 to the address 0x0. Reading characters works in quite a similar way.

The simulator also has some monitoring capacities to help in the debugging process and can simulate hardware interrupts.

Assembler

Writing machine code by hand is doable, especially with an ISA as simple as Reflet. Indeed, the opcodes being either 4 or 8 bits, they are quite easy to read or write in hexadecimal. But for our sanity's sake and to use some labels and macro, I made an assembler/linker that converts the assembly language into Reflet machine code.

This assembler is not very well written, it needs some optimizations and it does not work with proper tokenizer-parser, instead, it works with a Let's read each lines-I will wing it technology stack.

Nevertheless, as writing macros for it is quite easy and it comes with very convenient default macros, it is perfectly usable as an assembler.

Hello world

Now that we have both an assembler and a simulator, we can write and execute a program. Let's write an Hello, world!:

label msg
@string Hello, world!
@rawbytes A D 0

label start
    set 15
    cpy R1 ; Length of the message to print in R1
    setlab msg
    cpy R2 ; Pointer to the character to print in R2
    label print-loop
        ; Loading a character from R2 and storing it in R3
        tbm
        load R2
        cpy R3
        tbm
        ; Function call made thanks to the `callf` macro
        callf putc
        ; Incrementing the pointer to the desired character
        set 1
        add R2
        cpy R2
        ; Decrementing the number of characters left to print
        set 1
        sub R1
        cc2
        cpy R1
        ; If the number of characters left to print is not 0, jump back to the start of the loop
        set 0
        eq R1
        cmpnot
        setlab print-loop
        jif
    quit

; Function that prints the character in R3
label putc
    ; Preparing data_tx address
    set 1
    cpy R4
    tbm
    ; Writing the content of R3 into data_tx
    read R3
    str R4
    ; Writing 0 into data_cmd
    set 0
    cpy R4
    str R4
    tbm
    ret

We can observe that the program does not make any assumption regarding the word size of the processor. Thus, we can compile it and run it for a Reflet processor of any word size.

Let's compile it and run it on an 8-bit simulated processor:

$ reflet-asm Hello.asm -o Hello.bin -word-size 8
$ reflet-sim Hello.bin
Hello, world!

Yay! It works. Let's have a look at the generated machine code:

00000000  41 53 52 4d 3c 2d 3b 2c  10 3d 11 3c 12 4c 4e 3e  |ASRM<-;,.=.<.LN>|
00000010  a2 13 4c 08 4e f0 3c 2b  3d 2c 3f 3c 2d 3b 2c 10  |..L.N.<+=,?<-;,.|
00000020  3d 11 3c 12 4c 4e 3e 42  13 4c 08 4e f0 3c 2b 3d  |=.<.LN>B.L.N.<+=|
00000030  2c 3e 48 65 6c 6c 6f 2c  20 77 6f 72 6c 64 21 0a  |,>Hello, world!.|
00000040  0d 00 1f 31 3c 2d 3b 2c  10 3d 11 3c 12 4c 4e 3e  |...1<-;,.=.<.LN>|
00000050  32 13 4c 08 4e f0 3c 2b  3d 2c 32 03 f2 33 03 3c  |2.L.N.<+=,2..3.<|
00000060  2d 3b 2c 10 3d 11 3c 12  4c 4e 3e 98 13 4c 08 4e  |-;,.=.<.LN>..L.N|
00000070  f0 3c 2b 3d 2c 0c 11 42  32 11 51 08 31 10 c1 01  |.<+=,..B2.Q.1...|
00000080  3c 2d 3b 2c 10 3d 11 3c  12 4c 4e 3e 5b 13 4c 08  |<-;,.=.<.LN>[.L.|
00000090  4e f0 3c 2b 3d 2c 09 0e  11 34 03 23 e4 10 34 e4  |N.<+=,...4.#..4.|
000000a0  03 0d                                             |..|

At first, it doesn't look like much but by taking a closer look, we can read the machine code. For example, we can see that the function putc starts at address 98 and we can read the instruction we rote in assembly. 0x11 is set 1, 0x34 is cpy R4, 0x03 is tbm...

Conclusion

Now that we have designed the ISA and that we have made tools to write machine code for it, the next step is to implement it in a hardware description language such as Verilog.

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