From fc3a0bbbe9d20299a1e26123dc901daf5f1b3fb6 Mon Sep 17 00:00:00 2001
From: CrazyEttin <>
Date: Thu, 17 Jun 2021 01:03:00 +0300
Subject: [PATCH] Add an error message to the bootloader in case the system is
 not found, fix two related bugs in readstr related to movement between lines
 and scrolling, and tidy the code up a bit.

---
 src/BOOT.ASM     | 23 +++++++++++++++---
 src/CMPSTR.INC   | 22 +++++++----------
 src/PRINTSTR.INC |  9 +++----
 src/READSTR.INC  | 63 ++++++++++++++++++++++++++++++++++++++----------
 src/SYSTEM.ASM   | 15 +++---------
 5 files changed, 85 insertions(+), 47 deletions(-)

diff --git a/src/BOOT.ASM b/src/BOOT.ASM
index 43b76e0..4f8f289 100644
--- a/src/BOOT.ASM
+++ b/src/BOOT.ASM
@@ -112,6 +112,20 @@ add di, ax
 pop cx
 loop search
 
+;Print an error message if the system is not found
+mov si, errormsg
+printerror:
+;Load a character
+lodsb
+;Check for the string end
+cmp al, 0x0
+je $
+;Print the character
+mov ah, 0xe
+int 0x10
+;Repeat
+jmp printerror
+
 ;Load the system entry
 loadfat:
 ;Load CX from the stack
@@ -189,10 +203,11 @@ boot:
 jmp 0x0:0x500
 
 ;Data
-bootdev db 0x0
-sysfile db "SYSTEM  BIN"
-cluster dw 0x0
-pointer dw 0x500
+bootdev  db 0x0
+sysfile  db "SYSTEM  BIN"
+errormsg db "System not found", 0xd, 0xa, 0x0
+cluster  dw 0x0
+pointer  dw 0x500
 
 ;Calculate the source arguments for loading data from the disk
 calcsource:
diff --git a/src/CMPSTR.INC b/src/CMPSTR.INC
index 014cdda..15a8110 100644
--- a/src/CMPSTR.INC
+++ b/src/CMPSTR.INC
@@ -6,35 +6,31 @@ cmpstr:
 push ax
 push bx
 
+;Compare the strings
 .loop:
-
-;Load characters
+;Load the current characters
 mov al, [si]
 mov bl, [di]
-
 ;Compare the characters
 cmp al, bl
-
 ;Check for difference
 jne .neq
-
 ;Check for the string end
 cmp al, 0x0
 je .eq
-
-;Repeat
+;Repeat for the next characters
 inc si
 inc di
 jmp .loop
 
-.neq:
-;Clear the carry flag
-clc
+;Set the carry flag
+.eq:
+stc
 jmp .done
 
-.eq:
-;Set the carry flag
-stc
+;Clear the carry flag
+.neq:
+clc
 
 .done:
 
diff --git a/src/PRINTSTR.INC b/src/PRINTSTR.INC
index 7f3af5c..67c534d 100644
--- a/src/PRINTSTR.INC
+++ b/src/PRINTSTR.INC
@@ -5,20 +5,17 @@ printstr:
 ;Store the initial registers in the stack
 push ax
 
+;Print the string
 .loop:
-
-;Load a character
+;Load the current character
 lodsb
-
 ;Check for the string end
 cmp al, 0x0
 je .done
-
 ;Print the character
 mov ah, 0xe
 int 0x10
-
-;Repeat
+;Repeat for the next character
 jmp .loop
 
 .done:
diff --git a/src/READSTR.INC b/src/READSTR.INC
index 16b7d42..3bb5fab 100644
--- a/src/READSTR.INC
+++ b/src/READSTR.INC
@@ -8,10 +8,10 @@ push bx
 push cx
 push dx
 
+;Setup
 ;Store the input length in the stack
 mov ah, 0
 push ax
-
 ;Initialise the destination with spaces
 mov cx, ax
 mov al, 0x20
@@ -19,9 +19,8 @@ rep stosb
 pop ax
 push ax
 sub di, ax
-
-;Initialise the cursor pointer
-mov bl, 0x1
+;Initialise the cursor pointer in BL and clear BH
+mov bx, 0x1
 
 .loop:
 
@@ -29,6 +28,7 @@ mov bl, 0x1
 mov ah, 0x0
 int 0x16
 
+;Check for special keys and non-printing characters
 ;Check for return
 cmp al, 0xd
 je .return
@@ -52,6 +52,7 @@ jle .loop
 cmp al, 0x7f
 je .loop
 
+;Store and print a character
 .character:
 ;Store the  character
 stosb
@@ -62,26 +63,30 @@ int 0x10
 inc bl
 jmp .loop
 
-.erase:
 ;Replace the cursor position with a space
+.erase:
 mov al, 0x20
 jmp .character
 
+;Move the cursor forward
 .space:
 call .nextchar
 inc di
 inc bl
 jmp .loop
 
+;Move the cursor backward
 .backspace:
 ;Check for the input beginning
 cmp bl, 0x1
 je .loop
+;Move the cursor
 call .prevchar
 dec di
 dec bl
 jmp .loop
 
+;Finish reading the string
 .return:
 
 ;Find and remove trailing spaces
@@ -126,40 +131,72 @@ pop ax
 
 ret
 
+;Move the cursor forward
+
+;Move forward within a line
 .nextchar:
 ;Get the cursor position
 mov ah, 0x3
 int 0x10
-;Move from the end of a line to the beginning of the next one
-cmp dl, [cpl]
+;Check for the end of the line
+cmp dl, 0x4f
 je .nextln
-;Move forward within a line
+;Move
 inc dl
 mov ah, 0x2
 int 0x10
 ret
+
+;Move to the beginning of the next line
 .nextln:
+;Check if the current line is the last on screen
+cmp dh, 0x18
+je .scroll
+;Move
 mov ah, 0x2
 inc dh
-mov dl, 0x1
+mov dl, 0x0
 int 0x10
 ret
 
+;Scroll the screen up by one line
+.scroll:
+;Scroll
+mov ah, 0x6
+mov al, 0x1
+mov bh, 0x7
+mov ch, 0x0
+mov cl, 0x0
+mov dh, 0x18
+mov dl, 0x4f
+int 0x10
+;Move to the beginning of the new line
+mov ah, 0x2
+mov bh, 0x0
+mov dl, 0x0
+int 0x10
+ret
+
+;Move the cursor backward
+
+;Move backward within a line
 .prevchar:
 ;Get the cursor position
 mov ah, 0x3
 int 0x10
-;Move from the beginning of a line to the end of the previous one
-cmp dl, 0x1
+;Check for the beginning of the line
+cmp dl, 0x0
 je .prevln
-;Move backward within a line
+;Move
 dec dl
 mov ah, 0x2
 int 0x10
 ret
+
+;Move to the end of the previous line
 .prevln:
 mov ah, 0x2
 dec dh
-mov dl, [cpl]
+mov dl, 0x4f
 int 0x10
 ret
diff --git a/src/SYSTEM.ASM b/src/SYSTEM.ASM
index be0ca14..3063924 100644
--- a/src/SYSTEM.ASM
+++ b/src/SYSTEM.ASM
@@ -26,13 +26,8 @@ mov sp, stack
 add sp, 0x100
 sti
 
-;Get the terminal width
-mov ah, 0xf
-int 0x10
-mov [cpl], ah
-
 ;Print a welcome message
-mov si, welcome
+mov si, welcomemsg
 call println
 
 loop:
@@ -98,13 +93,11 @@ jnc .error
 call keycode
 jmp loop
 .error:
-mov si, error
+mov si, errormsg
 call println
 jmp loop
 
-cpl db 0x0
-
-welcome db 0xd, 0xa, "Welcome to EttinOS!", 0xd, 0xa, 0x0
+welcomemsg db 0xd, 0xa, "Welcome to EttinOS!", 0xd, 0xa, 0x0
 
 prompt db "> ", 0x0
 
@@ -115,7 +108,7 @@ cmd:
 .help    db "help", 0x0
 .keycode db "keycode", 0x0
 
-error db "Unknown command", 0x0
+errormsg db "Unknown command", 0x0
 
 buffer times 0xff db 0x0