doc: add glue code section

doc/README.md

@@ -1,10 +1,10 @@
-# User Guide
+# Collapse OS documentation
 
-This collection of document is intended to be a user guide, not assembly
-instructions. It is therefore assumed that you have a machine with Collapse OS
-properly running.
+## Assembly guide
 
-## Table of Contents
+* [Writing the glue code](glue-code.md)
+
+## User guide
 
 * [The shell](shell.md)
 * [Load code in RAM and run it](load-run-code.md)

doc/glue-code.md

@@ -0,0 +1,120 @@
+# Writing the glue code
+
+Collapse OS is not an OS, it's a meta OS. It supplies parts that you're expected
+to glue together in a "glue code" asm file. Here is what a minimal glue code
+for a shell on a Classic [RC2014][rc2014] with an ACIA link would look like:
+
+
+    ; The RAM module is selected on A15, so it has the range 0x8000-0xffff
+    RAMSTART	.equ	0x8000
+    RAMEND		.equ	0xffff
+    ACIA_CTL	.equ	0x80	; Control and status. RS off.
+    ACIA_IO		.equ	0x81	; Transmit. RS on.
+
+        jr init
+
+    ; interrupt hook
+    .fill	0x38-$
+        jp aciaInt
+
+    init:
+        di
+        ; setup stack
+        ld hl, RAMEND
+        ld sp, hl
+        im 1
+        call aciaInit
+        call shellInit
+        ei
+        jp	shellLoop
+
+    #include "core.asm"
+    ACIA_RAMSTART	.equ	RAMSTART
+    #include "acia.asm"
+    SHELL_RAMSTART	.equ	ACIA_RAMEND
+    .define SHELL_GETC	call aciaGetC
+    .define SHELL_PUTC	call aciaPutC
+    .define SHELL_IO_GETC	call aciaGetC
+    SHELL_EXTRA_CMD_COUNT .equ 0
+    #include "shell.asm"
+
+Once this is written, building it is easy:
+
+    scas -o collapseos.bin -I /path/to/parts glue.asm
+
+## Platform constants
+
+The upper part of the code contains platform-related constants, information
+related to the platform you're targeting. You might want to put it in an
+include file if you're writing multiple glue code that targets the same machine.
+
+In all cases, `RAMSTART` are necessary. `RAMSTART` is the offset at which
+writable memory begins. This is where the different parts store their
+variables.
+
+`RAMEND` is the offset where writable memory stop. This is generally
+where we put the stack, but as you can see, setting up the stack is the
+responsibility of the glue code, so you can set it up however you wish.
+
+`ACIA_*` are specific to the `acia` part. Details about them are in `acia.asm`.
+If you want to manage ACIA, you need your platform to define these ports.
+
+## Header code
+
+Then comes the header code (code at `0x0000`), a task that also is in the glue
+code's turf. `jr init` means that we run our `init` routine on boot.
+
+`jp aciaInt` at `0x38` is needed by the `acia` part. Collapse OS doesn't dictate
+a particular interrupt scheme, but some parts might. In the case of `acia`, we
+require to be set in interrupt mode 1.
+
+## Includes
+
+This is the most important part of the glue code and it dictates what will be
+included in your OS. Each part is different and has a comment header explaining
+how it works, but there are a couple of mechanisms that are common to all.
+
+### Defines
+
+Parts can define internal constants, but also often document a "Defines" part.
+These are constant that are expected to be set before you include the file.
+
+See comment in each part for details.
+
+### RAM management
+
+Many parts require variables. They need to know where in RAM to store these
+variables. Because parts can be mixed and matched arbitrarily, we can't use
+fixed memory addresses.
+
+This is why each part that needs variable define a `<PARTNAME>_RAMSTART`
+constant that must be defined before we include the part.
+
+Symmetrically, each part define a `<PARTNAME>_RAMEND` to indicate where its
+last variable ends.
+
+This way, we can easily and efficiently chain up the RAM of every included part.
+
+### Tables grafting
+
+A mechanism that is common to some parts is "table grafting". If a part works
+on a list of things that need to be defined by the glue code, it will place a
+label at the very end of its source file. This way, it becomes easy for the
+glue code to "graft" entries to the table. This approach, although simple and
+effective, only works for one table per part. But it's often enough.
+
+For example, to define extra commands in the shell:
+
+    [...]
+    SHELL_EXTRA_CMD_COUNT .equ 2
+    #include "shell.asm"
+    .dw myCmd1, myCmd2
+    [...]
+
+### Initialization
+
+Then, finally, comes the `init` code. This can be pretty much anything really
+and this much depends on the part you select. But if you want a shell, you will
+usually end it with `shellLoop`, which never returns.
+
+[rc2014]: https://rc2014.co.uk/

parts/shell.asm

@@ -72,6 +72,8 @@ shellInit:
 .welcome:
 	.db	"Collapse OS", ASCII_CR, ASCII_LF, "> ", 0
 
+; Inifite loop that processes input. Because it's infinite, you should jump
+; to it rather than call it. Saves two precious bytes in the stack.
 shellLoop:
 	; First, let's wait until something is typed.
 	SHELL_GETC
@@ -116,6 +118,7 @@ shellLoop:
 	ld	hl, .prompt
 	call	printstr
 	jr	shellLoop
+	; no ret because we never return
 
 .prompt:
 	.db	"> ", 0