clear the carry flag before calling BIOS functions

caught a bug where SetTextMode was setting mono text not 16-color mode

include/BIOS/func/E820_memory_map.nasm

@@ -53,6 +53,7 @@ GetMemoryMap:
 .loop_L1:
     mov eax, 0x0000E820                     ; select 0xE820 function
     mov edx, 0x534D4150                     ; 'SMAP' magic
+    clc                                     ; clear carry
     int 0x15
     jc GetMemoryMap.error
     cmp eax, 0x534D4150

include/BIOS/func/a20enable.nasm

@@ -105,6 +105,7 @@ EnableA20:
     je EnableA20.endp         ; A20 is already enabled
 
     mov ax, 0x2403
+    clc                             ; clear carry
     int 0x15
     jc EnableA20.do_fallback_a20    ; carry = error...not supported?
     cmp ah, 0
@@ -121,6 +122,7 @@ EnableA20:
     ERROR STAGE2_A20_FAILED
 .do_bios_a20:
     mov ax, 0x2401
+    clc                             ; clear carry
     int 0x15
     jmp EnableA20.a20_check
 .do_fast_a20:

include/BIOS/func/ext_read.nasm

@@ -111,6 +111,7 @@ read_disk_raw:
     mov ah, 0x42                ; call #
     mov dl, byte [bp + 14]      ; drive #             
 
+    clc                         ; clear carry
     int 0x13
     jnc .endf
 

include/BIOS/func/video.nasm

@@ -24,12 +24,14 @@ SetTextMode:
     pushf
 .func:
     xor ah, ah                  ; Set Video mode BIOS function
-    mov al, 0x02                ; 16 color 80x25 Text mode
+    mov al, 0x03                ; 16 color 80x25 Text mode
     int 0x10                    ; Call video interrupt
+                                ; TODO: check for carry and clear carry before call
 
     mov ah, 0x05                ; Select active display page BIOS function
     xor al, al                  ; page 0
     int 0x10                    ; call video interrupt
+                                ; TODO: check for carry and clear carry before call
 .endp:
     popf
     __CDECL16_EXIT
@@ -51,5 +53,102 @@ disable_cursor:
     __CDECL16_EXIT
     ret
 
+; Prints a C-Style string (null terminated) using BIOS vga teletype call
+; void PrintString(char* buf)
+ALIGN 4, db 0x90
+PrintString:
+    __CDECL16_ENTRY
+    mov di, [bp + 4]    ; first arg is char[] 
+.str_len:
+    xor cx, cx         ; ECX = 0
+    not cx             ; ECX = -1 == 0xFFFF
+    xor ax, ax         ; search for al = 0x0
+
+    cld
+    repne scasb        ; deincrement cx while searching for al
+
+    not cx             ; the inverse of a neg number = abs(x) - 1
+    dec cx             ; CX contains the length of the string - nul byte at end
+.print:
+    mov si, [bp + 4]            ; source string
+.print_L0:
+    movzx ax, byte [si]
+    push ax
+    call PrintCharacter
+    add sp, 0x2
+
+    inc si
+    dec cx
+
+    jcxz PrintString.endp
+    jmp PrintString.print_L0    ; Fetch next character from string
+.endp:
+    __CDECL16_EXIT
+    ret                         ; Return from procedure
+
+; Prints a single character
+; void PrintCharacter(char c);
+ALIGN 4, db 0x90
+PrintCharacter:
+    __CDECL16_ENTRY
+.func:
+    movzx ax, byte [bp + 4]     ; AL = character c
+    mov ah, 0x0E                ; INT 0x10, AH=0x0E call
+    mov bx, 0x0007              ; BH = page no. BL =Text attribute 0x07 is lightgrey font on black background
+    int 0x10                    ; call video interrupt
+                                ; TODO: check for carry and clear carry before call
+.endp:
+    __CDECL16_EXIT
+    ret
+
+; TODO: fix the prolog, epilog and stack usage to confirm with cdecl16
+; prints the hex representation of of val
+; void PrintDWORD(uint32_t val);
+ALIGN 4, db 0x90
+PrintDWORD:
+    __CDECL16_ENTRY
+.func: 
+    mov si, IntToHex_table
+    mov ebx, 16     ; base-16
+
+    mov eax, dword [bp + 4]     ;val
+
+    xor edx, edx
+    xor cx, cx
+.next_digit:
+    div ebx            ; dividend in edx:eax -> quotient in eax, remainder in edx
+    push dx            ; save remainder
+    inc cx
+
+    xor dx, dx
+    test eax, eax
+    jnz PrintDWORD.next_digit
+
+.zero_pad:
+    cmp cx, 0x0008
+    je PrintDWORD.print_stack
+    xor ax, ax
+    push ax
+    inc cx
+    jmp PrintDWORD.zero_pad
+
+.print_stack:
+    pop bx
+    dec cx
+    push cx
+
+    movzx ax, byte [bx+si+0]    ; bx = index into Hex lookup table
+    push ax
+    call PrintCharacter
+    add sp, 0x2
+
+    pop cx
+
+    jcxz PrintDWORD.endp
+    jmp PrintDWORD.print_stack
+
+.endp:
+    __CDECL16_EXIT
+    ret
 %endif
 %define __INC_VIDEO

src/mbr/mbr.nasm

@@ -96,6 +96,8 @@ main:
         mov ah, 0x41
         mov bx, 0x55AA
         mov dl, 0x80
+        
+        clc
         int 0x13
         jnc main.find_active
         ERROR MBR_ERROR_NO_INT32E                         ; no extended function support

src/stage2/stage2.nasm

@@ -191,102 +191,7 @@ hcf:
 ;
 ; ##############################
 
-; Prints a C-Style string (null terminated) using BIOS vga teletype call
-; void PrintString(char* buf)
-ALIGN 4, db 0x90
-PrintString:
-    __CDECL16_ENTRY
-    mov di, [bp + 4]    ; first arg is char[] 
-.str_len:
-    xor cx, cx         ; ECX = 0
-    not cx             ; ECX = -1 == 0xFFFF
-    xor ax, ax         ; search for al = 0x0
 
-    cld
-    repne scasb        ; deincrement cx while searching for al
-
-    not cx             ; the inverse of a neg number = abs(x) - 1
-    dec cx             ; CX contains the length of the string - nul byte at end
-.print:
-    mov si, [bp + 4]            ; source string
-.print_L0:
-    movzx ax, byte [si]
-    push ax
-    call PrintCharacter
-    add sp, 0x2
-
-    inc si
-    dec cx
-
-    jcxz PrintString.endp
-    jmp PrintString.print_L0    ; Fetch next character from string
-.endp:
-    __CDECL16_EXIT
-    ret                         ; Return from procedure
-
-; Prints a single character
-; void PrintCharacter(char c);
-ALIGN 4, db 0x90
-PrintCharacter:
-    __CDECL16_ENTRY
-.func:
-    movzx ax, byte [bp + 4]     ; AL = character c
-    mov ah, 0x0E                ; INT 0x10, AH=0x0E call
-    mov bx, 0x0007              ; BH = page no. BL =Text attribute 0x07 is lightgrey font on black background
-    int 0x10                    ; call video interrupt
-.endp:
-    __CDECL16_EXIT
-    ret
-
-; TODO: fix the prolog, epilog and stack usage to confirm with cdecl16
-; prints the hex representation of of val
-; void PrintDWORD(uint32_t val);
-ALIGN 4, db 0x90
-PrintDWORD:
-    __CDECL16_ENTRY
-.func: 
-    mov si, IntToHex_table
-    mov ebx, 16     ; base-16
-
-    mov eax, dword [bp + 4]     ;val
-
-    xor edx, edx
-    xor cx, cx
-.next_digit:
-    div ebx            ; dividend in edx:eax -> quotient in eax, remainder in edx
-    push dx            ; save remainder
-    inc cx
-
-    xor dx, dx
-    test eax, eax
-    jnz PrintDWORD.next_digit
-
-.zero_pad:
-    cmp cx, 0x0008
-    je PrintDWORD.print_stack
-    xor ax, ax
-    push ax
-    inc cx
-    jmp PrintDWORD.zero_pad
-
-.print_stack:
-    pop bx
-    dec cx
-    push cx
-
-    movzx ax, byte [bx+si+0]    ; bx = index into Hex lookup table
-    push ax
-    call PrintCharacter
-    add sp, 0x2
-
-    pop cx
-
-    jcxz PrintDWORD.endp
-    jmp PrintDWORD.print_stack
-
-.endp:
-    __CDECL16_EXIT
-    ret
 
 ; ##############################
 ;