Add BootROM support.

XTMax/Drivers/BootROM/.gitignore

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+bootrom
+bootrom.com

XTMax/Drivers/BootROM/.vscode/tasks.json

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+{
+    // See https://go.microsoft.com/fwlink/?LinkId=733558
+    // for the documentation about the tasks.json format
+    "version": "2.0.0",
+    "tasks": [
+        {
+            "label": "Build with NASM",
+            "type": "shell",
+            "command": ".\\build.cmd",
+            "options": {
+                "cwd": "${workspaceFolder}"
+            },
+            "problemMatcher": [],
+            "group": {
+                "kind": "build",
+                "isDefault": true
+            }
+        }
+    ]
+}

XTMax/Drivers/BootROM/bootrom.asm

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+bits 16
+
+%ifndef AS_COM
+;
+; BIOS will look for the AA55 signature between C8000-DFFFF in 2KB increments.
+; We choose an address in that range.
+;
+org 0xce000
+dw 0AA55h
+db 4    ; times 512 bytes
+
+%else
+;
+; Building as a COM file for testing.
+;
+org 0x100
+
+%endif
+
+entry:
+%ifndef AS_COM
+    mov ax, 0xc000
+    mov ds, ax
+    mov es, ax
+%endif
+
+    mov si, welcome
+    call print_string
+
+
+%ifndef AS_COM
+    retf
+%else
+;
+; DOS exit program.
+;
+    mov ah, 0x4c
+    xor al, al
+    int 0x21
+%endif
+
+;
+; Utilities
+;
+
+print_string:
+    push bx
+    xor bx,bx
+.loop:
+    lodsb
+    or al, al
+    jz .done
+    mov ah, 0x0E
+    int 0x10
+    jmp .loop
+.done:
+    pop bx
+    ret
+
+;
+; Strings
+;
+
+welcome db 'BootROM for XTMax v0.1', 0x0D, 0x0A, 0
+
+%ifndef AS_COM
+;
+; Pad to 2KB. We will try to keep our ROM under that size.
+;
+times 2047-($-$$) db 0
+db 0    ; will be used to complete the checksum.
+%endif

XTMax/Drivers/BootROM/build.cmd

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+..\Driver_Build_Tools\NASM\nasm.exe -f bin -o bootrom.com -DAS_COM .\bootrom.asm
+..\Driver_Build_Tools\NASM\nasm.exe -f bin -o bootrom .\bootrom.asm & python checksum.py

XTMax/Drivers/BootROM/checksum.py

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+# https://stackoverflow.com/questions/53808694/how-do-i-format-a-python-list-as-an-initialized-c-array
+def to_c_array(values, ctype="float", name="table", formatter=str, colcount=8):
+    # apply formatting to each element
+    values = [formatter(v) for v in values]
+
+    # split into rows with up to `colcount` elements per row
+    rows = [values[i:i+colcount] for i in range(0, len(values), colcount)]
+
+    # separate elements with commas, separate rows with newlines
+    body = ',\n    '.join([', '.join(r) for r in rows])
+
+    # assemble components into the complete string
+    return '{} {}[] = {{\n    {}}};'.format(ctype, name, body)
+
+with open("bootrom", "rb") as f:
+    b = bytearray(f.read())
+# compute the checksum and store a byte to
+b[-1] = 256 - sum(b[0:-1]) & 0xff
+with open("bootrom", "wb") as f:
+    f.write(b)
+with open("../../Code/XTMax/bootrom.h", "w") as f:
+    f.write("#define BOOTROM_ADDR 0xCE000\n")
+    f.write(to_c_array(b, ctype="unsigned char", name="BOOTROM", colcount=16))