Update the README and comments with detailed compatibility notes

I looked at every interrupt used and noted which PC models supported it.
Turns out it's closer than I thought to working on the original 5150 PC.

I learned a lot about the history of PC models today.

Also
 - Refactored the BIOS_PRINT_STRING constant in the bootsector to not
   include the al setting for what to do with the cursor
 - Fixed some spelling mistakes

README.md

@@ -17,28 +17,40 @@ exactly that.
 
 ## Compatibility
 
-This code will work on any x86 CPU made between 2020 and 1981 with EGA support.
-So any modern x86 PC with BIOS support. I believe the earliest machine it would
-work on is the original 1981 IBM PC with a 1984 EGA card expansion.
+This code should run correctly on any x86 PC compatible machine made between
+1984 and 2020. I believe the earilest machine that will run it is the IBM PC AT
+with the EGA card expansion released in October 1984.
 
 If it doesn't boot on your machine that supports BIOS, please let me know. I'd
 love to find out why.
 
 ### Compatibility details
 
 This code is 16 bit real mode x86 assembly which Intel has kept available on all
-x86 CPUs since the original 8086 processor from 1976.
+x86 CPUs since the original 8086 processor from 1976. The assembly code itself
+is compatible with the 8086.
 
-It also depends on the IBM PC BIOS hardware interface standard from 1981 which
-modern computers still implement. Sadly Intel has partially ended this
+It also depends on the IBM PC BIOS hardware interface standard started in 1981
+which modern computers still implement. Sadly Intel has partially ended this
 incredible nealy 40 year backwards compatibility story by officially
 [ending support for BIOS](https://www.bleepingcomputer.com/news/hardware/intel-plans-to-end-legacy-bios-support-by-2020/)
 as of 2020, so now there are some machines on the market that only support UEFI
 booting.
 
-Finally it depends on EGA, the Enhanced Graphics Adapter card API, because it
-uses the 80x25 16 colors text video mode which was added with that card. This is
-the predicessor to CGA and VGA.
+The latest BIOS call used here is int 0x10 with ah = 0x13 for printing a string.
+This was introduced with PC AT although the earlier XT machines got this feature
+later with a BIOS update. Additionally it uses BIOS functions to set the
+overscan color and blink bit behavior which where added with the EGA card, so it
+is technically not compatible with the CGA card although it will mostly work.
+
+If we had a replacement for BIOS print string based on the print character
+interrupts, technically it could be compatible with the original PC from 1981.
+However the lisp interpreter code is more than the 8 sectors per track the PC
+supported and the bootsector code expects to have multi-track read support which
+the PC didn't have originially (maybe you could get a BIOS that does have it).
+
+Additionally the text editor requires 256k of RAM which was the maximum possible
+PC configuration.
 
 ## Why?
 

bootloader/bootsect.asm

@@ -31,15 +31,26 @@
 ; check here because nasm requires that to go after NUM_EXTRA_SECTORS is
 ; defined.
 
-%define BIOS_PRINT_STRING 0x1301
-%define BIOS_PRINT_CHAR 0x0E
+; NOTE: The write string BIOS call only works for:
+; PC XT BIOS dated 1/10/86 and after, AT, EGA, PC Convertible,
+; and Personal System/2 products
+%define BIOS_PRINT_STRING 0x13
+%define BIOS_PRINT_CHAR 0x0E ; Works on the original PC
 %define QEMU_DEBUG_PORT 0xE9
 
 %include "bootloader/bootsect-header.asm"
 
 mov [BOOT_DISK], dl ; Save the boot disk number
 
+; NOTE: the PC with CGA card has int 0x10 ah = 0 to 0xF the rest are ignored
+
+; TODO: Do some sort of video mode detection. Error if only mono is supported
+; unless it turns out the attribute bytes are somehow compatible.
+; It may not be necessary to do the full detection procedure although it would
+; be nice to know if we have EGA support at least.
+
 ; Set video mode, 16 color 80x25 chars
+; This is supported by CGA cards and PCjr
 ;
 ; The IBM BIOS manual describes a long procedure for determining which video
 ; modes are supported and all the possible options for supporting both mono and
@@ -49,13 +60,16 @@ mov ax, 0x0003
 int 0x10
 
 ; Make the 8th bit of the colors bg-intensity instead of blink
+; This works on EGA and VGA
+; TODO: support doing the out port address for CGA and detect CGA
 mov ax, 0x1003
 mov bl, 0
 int 0x10
 
 ; Make the 4th bit of the colors fg-intensity instead of font selection
-; Use block 0 for the font
-; Apparently this is the default in VGA so maybe we don't need it
+; Use block 0 for the font.
+; This means we only have 256 characters instead of 512.
+; Apparently this is the default in EGA and VGA so maybe we don't need it
 mov ax, 0x1103
 mov bl, 0
 int 0x10
@@ -68,6 +82,8 @@ mov cx, 0x0607
 int 0x10
 
 ; Set the keyboard repeat speed
+; For AT BIOS dated 11/15/85 and after, PC XT Model 286, and Personal System/2 products
+; The original PC ignores this
 mov ax, 0x0305
 ;mov bx, 0x0107 ; 500 ms delay before repeat ; 16 characters per second
 mov bx, 0x0100 ; 500 ms delay before repeat ; 30 characters per second
@@ -81,9 +97,14 @@ mov al, NUM_EXTRA_SECTORS ; number of sectors to read
 test al, al
 je start_
 
+; TODO: do the proper thing and detect the max sectors per track and don't
+; assume multi-track read support.
+
 ; Load the code from the extra sectors
 ; Try to do the full read every time to take advantage of possible BIOS support
 ; for multi-track reads if NUM_EXTRA_SECTORS > 63
+;
+; This worked on the original PC, but the max sectors to read was 8 (max track was 39, head 1)
 mov ah, 0x02
 mov bx, SECTOR_SIZE ; es:bx is address to write to. es = cs, so write directly after the boot sector
 mov dl, [BOOT_DISK] ; Drive number
@@ -96,8 +117,13 @@ jnc start_ ; if there was no error, jump to the loaded user code
 
 push ax ; push the error code
 
+; TODO: maybe make a print_string function that can also write to QEMU
+; maybe I could even have a fallback for if ah = 13 isn't available, but I'm not
+; totally sure how to detect. Maybe just always replace it for CGA cards.
+
 ; Print the error message
-mov ax, 0x1301 ; Write String, move cursor mode in al
+mov ah, BIOS_PRINT_STRING
+mov al, 1 ; no attr bytes, move cursor
 mov bp, disk_error_msg ; String pointer in es:bp (es is at code start from bootsect-header.asm)
 mov cx, disk_error_msg_len ; String length
 xor dx, dx ; top left
@@ -107,7 +133,8 @@ int 0x10
 pop cx ; pop the error code
 call print_hex ; print the error code
 
-mov ax, 0x1301 ; Write String, move cursor mode in al
+mov ah, BIOS_PRINT_STRING
+mov al, 1 ; no attr bytes, move cursor
 mov bp, retry_msg ; String pointer in es:bp (es is at code start from bootsect-header.asm)
 mov cx, retry_msg_len ; String length
 mov dx, 0x0100 ; second line; left edge
@@ -121,6 +148,7 @@ cmp byte [READ_RETRIES], 0
 je .no_more_tries
 
 ; Reset the disk and retry
+; Works on the original PC
 dec byte [READ_RETRIES]
 xor ax, ax
 mov dl, [BOOT_DISK] ; Drive number
@@ -134,6 +162,7 @@ jmp $ ; stop forever
 ; Only accepts 0x0 <= al <= 0xF, anything else is garbage output
 ; e.g. al = 12 prints "C"
 ; clobbers ax, and bx
+; Works on the original PC
 print_hex_char:
   mov ah, BIOS_PRINT_CHAR ; Scrolling teletype BIOS routine (used with int 0x10)
   xor bx, bx ; Clear bx. bh = page, bl = color
@@ -159,6 +188,7 @@ print_hex_char:
 
 ; cx = two bytes to write at current cursor
 ; clobbers ax, and bx
+; Works on the original PC
 print_hex:
   ; Nibble 0 (most significant)
   mov al, ch

text-editor.asm

@@ -28,6 +28,9 @@
 %define END_ROW 0x16
 %define END_COL 0x4B
 
+; TODO: detect the RAM and give an error if we don't have 256k which is 0x40000
+; which is exactly the 4 segments we use (the 4th is the code segment)
+
 ; Segment register value (so the actual start is at the address 0x10*this)
 ; This is the first sector after the editor's code
 ;
@@ -76,6 +79,7 @@
 ; configured gdb to breakpoint
 start_:
 
+; TODO: CGA support
 ; Set the overscan color to the border color
 mov ax, 0x1001
 mov bh, OVERSCAN_COLOR
@@ -103,7 +107,7 @@ int 0x10
 
 ; Memory map:
 ;
-; Each segment gets a full non-overlaping 64k block of memory and we don't move
+; Each segment gets a full non-overlapping 64k block of memory and we don't move
 ; the segment so 64k is the limit for each part.
 ;
 ; SS: 0x050
@@ -117,7 +121,7 @@ int 0x10
 ;   - Extra memory passed to the plugin program that runs the code you typed.
 ;     Only set when ctrl+D is pressed and passed to the plugin.
 ;   - Normally the default for memory address reads and our bootloader leaves
-;     this set to CODE_SEGMENT for that. But this code alwasy uses cs: to
+;     this set to CODE_SEGMENT for that. But this code always uses cs: to
 ;     reference strings stored in the binary.
 
 ; --- typing_loop global register variables ---
@@ -133,7 +137,7 @@ int 0x10
 ; Note: We must have at least one char of gap i.e. si-di >= 1 at all times
 ;       (because we use it as scratch space when printing sometimes)
 ;
-; Additonally cx,bp are callee save while ax,bx are caller save (clobbered)
+; Additionally cx,bp are callee save while ax,bx are caller save (clobbered)
 mov ax, USER_CODE_LOC
 mov es, ax
 mov ax, COMPILER_DATA_LOC
@@ -1395,7 +1399,8 @@ print_line:
   ; dx is the cursor position to print at
   xor bh, bh ; page number 0
   mov bl, MAIN_COLOR
-  mov ax, 0x1301     ; Write String, with moving the cursor
+  mov ah, BIOS_PRINT_STRING  ; Write String
+  mov al, 1 ; no attr bytes, move the cursor
   int 0x10
   ret
 
@@ -1608,7 +1613,8 @@ print_error:
   mov dx, MAIN_TOP_LEFT-0x0100 ; one line above the top left corner
   xor bh, bh ; page number 0
   mov bl, ERROR_COLOR
-  mov ax, 0x1301 ; write string without moving the cursor
+  mov ah, BIOS_PRINT_STRING
+  mov al, 0 ; no attr bytes, don't move the cursor
   int 0x10
 
   ; reset the es segment