Move the system stack to the end of the address space, make the shell ignore empty input, and tweak printstr and readstr a bit.

README.MD

@@ -40,9 +40,11 @@ space and backspace keys move the cursor.
 Programming
 -----------
 
-EttinOS has a flat address space of 64 KiB with the data, stack, and
+EttinOS has a flat address space of 64 KiB. The data, stack, and
-extra segments set at the beginning of the RAM. Programs are loaded at
+extra segments are set at the beginning of the RAM and the system stack
-address 0x2000.
+at the end of the address space. Programs are loaded at address 0x2000.
+The stack is reset back to the end of the address space after a program
+has finished running.
 
 System calls:
   * Interrupt 0x20: Return to the shell.

src/BOOT.ASM

@@ -60,8 +60,7 @@ mov ss, ax
 mov es, ax
 ;Set up the stack
 cli
-mov sp, stack
+mov sp, 0x0
-add sp, 0x100
 sti
 ;Store the boot drive number
 mov [bootdrive], dl
@@ -75,7 +74,7 @@ add ax, [bootsectors]
 push ax
 call calcsource
 ;Set the destination
-mov si, 0x7f00
+mov si, buffer
 mov bx, si
 ;Set the size
 push dx
@@ -92,7 +91,7 @@ int 0x13
 
 ;Search the root for the system entry
 ;Set DI to the root
-mov di, 0x7f00
+mov di, buffer
 ;Initialise the search loop
 mov cx, word [rootentries]
 mov ax, 0x0
@@ -106,7 +105,7 @@ rep cmpsb
 je loadentry
 ;Set DI to the next entry
 add ax, 0x20
-mov di, 0x7f00
+mov di, buffer
 add di, ax
 ;Load CX from the stack
 pop cx
@@ -137,7 +136,7 @@ mov word [cluster], ax
 mov ax, 0x1
 call calcsource
 ;Set the destination
-mov di, 0x7f00
+mov di, buffer
 mov bx, di
 ;Set the size
 mov ax, [sectorsperfat]
@@ -176,7 +175,7 @@ mov bx, 0x3
 mul bx
 mov bx, 0x2
 div bx
-mov si, 0x7f00
+mov si, buffer
 add si, ax
 mov ax, word [ds:si]
 or dx, dx
@@ -198,8 +197,12 @@ mul word [clustersize]
 add word [pointer], ax
 jmp loadcluster
 
-;Boot the system
+;Clear the stack and boot the system
 boot:
+;Clear
+pop cx
+pop bx
+;Boot
 jmp 0x0:0x500
 
 ;Data
@@ -236,4 +239,5 @@ times 0x1fe-($-$$) db 0x0
 ;Boot signature
 dw 0xaa55
 
-stack:
+;File system buffer
+buffer:

src/LOADF.INC

@@ -76,8 +76,7 @@ add ax, [.bootsectors]
 push ax
 call .calcsource
 ;Set the destination
-mov si, stack
+mov si, buffer
-add si, 0x100
 mov bx, si
 ;Set the size
 push dx
@@ -94,8 +93,7 @@ int 0x13
 
 ;Search the root for the file entry
 ;Set DI to the root
-mov di, stack
+mov di, buffer
-add di, 0x100
 ;Initialise the search loop
 mov cx, word [.rootentries]
 mov ax, 0x0
@@ -109,8 +107,7 @@ rep cmpsb
 je .loadentry
 ;Set DI to the next entry
 add ax, 0x20
-mov di, stack
+mov di, buffer
-add di, 0x100
 add di, ax
 ;Load CX from the stack
 pop cx
@@ -134,8 +131,7 @@ mov word [.cluster], ax
 mov ax, 0x1
 call .calcsource
 ;Set the destination
-mov di, stack
+mov di, buffer
-add di, 0x100
 mov bx, di
 ;Set the size
 mov ax, [.sectorsperfat]
@@ -174,8 +170,7 @@ mov bx, 0x3
 mul bx
 mov bx, 0x2
 div bx
-mov si, stack
+mov si, buffer
-add si, 0x100
 add si, ax
 mov ax, word [ds:si]
 or dx, dx
@@ -200,7 +195,7 @@ jmp .loadcluster
 .clearcarry:
 clc
 
-;Clear left over values from the stack
+;Clear the stack
 .clearstack:
 pop cx
 pop bx
@@ -215,7 +210,7 @@ pop cx
 pop bx
 pop ax
 
-;Set AL to 0x1 if there was an error and to 0x0 otherwise
+;Set AL to 0x1 if there was an error and to 0x0 otherwise and return
 jc .setal
 mov al, 0x0
 iret

src/PRINTSTR.INC

@@ -4,6 +4,7 @@ printstr:
 
 ;Store the initial registers in the stack
 push ax
+push si
 
 ;Print the string
 .loop:
@@ -21,6 +22,7 @@ jmp .loop
 .done:
 
 ;Load the initial registers from the stack
+pop si
 pop ax
 
 iret

src/READSTR.INC

@@ -7,6 +7,7 @@ push ax
 push bx
 push cx
 push dx
+push di
 
 ;Setup
 ;Store the input length in the stack
@@ -124,6 +125,7 @@ jmp .findend
 .done:
 
 ;Load the initial registers from the stack
+pop di
 pop dx
 pop cx
 pop bx

src/SYSTEM.ASM

@@ -34,12 +34,6 @@ iret
 
 start:
 
-;Setup
-;Set up the stack
-cli
-mov sp, stack
-add sp, 0x100
-sti
 ;Set up the interrupt vectors
 ;Interrupt 0x20 offset
 mov ax, int0x20
@@ -63,10 +57,9 @@ int 0x21
 
 shell:
 
-;Re-set up the stack
+;Re-set the stack
 cli
-mov sp, stack
+mov sp, 0x0
-add sp, 0x100
 sti
 
 ;Prompt for and read a command
@@ -76,11 +69,14 @@ mov ah, 0x0
 int 0x21
 ;Read
 mov di, input
-mov al, 0xff
+mov al, 0x4e
 mov ah, 0x3
 int 0x21
 
 ;Load an execute the program
+;Check for an empty command
+cmp byte [input], 0x0
+jz shell
 ;Load
 mov bx, 0x2000
 mov si, input
@@ -109,7 +105,7 @@ jmp shell
 welcomemsg db 0xd, 0xa, "Welcome to EttinOS!", 0xd, 0xa, 0x0
 prompt     db "> ", 0x0
 errormsg   db "Unknown command", 0x0
-input      times 0xff db 0x0
+input      times 0x4e db 0x0
 crlf       db 0xd, 0xa, 0x0
 
 ;Print a CRLF
@@ -124,4 +120,5 @@ int 0x21
 pop si
 ret
 
-stack:
+;File system buffer
+buffer: