[kernel] Fix reading from empty board buffer (sf63.2)

Syscall 63.2, reading from the board buffer, was overwriting registers
edx and ebp when the buffer was empty. Now it sets eax and ebx according
to the kernel API.

Because of this ancient bug BOARD checks the status of sf63.2 syscall
this way:
        mcall   63, 2
        cmp     ebx, 1
        jne     no_data
        ; when the buffer is empty, ebx is untouched, i.e. still 2, haha
        ; edx and ebp are destroyed, the code is lucky not to use them

By the way, the bug was found using umka tool.

git-svn-id: svn://kolibrios.org@9897 a494cfbc-eb01-0410-851d-a64ba20cac60

kernel/trunk/kernel.asm

@@ -3393,7 +3393,6 @@ if 1
 align 4
 delay_hs:     ; delay in 1/100 secs
 ; ebx = delay time
-
         pushad
         push    ebx
         xor     esi, esi
@@ -4226,24 +4225,23 @@ end if
         pop     ebx eax
         ret
 
-@@:
-        mov     [esp + SYSCALL_STACK.eax], ecx
-        mov     [esp+20], ecx
-        jmp     .ret
-
 .read:
         cmp     eax, 2
         jne     .ret
-        test    ecx, ecx
-        jz      @b
         add     esp, 8  ; returning data in ebx and eax, so no need to restore them
+        test    ecx, ecx
+        jnz     @f
+        mov     [esp + SYSCALL_STACK.eax], ecx
+        mov     [esp + SYSCALL_STACK.ebx], ecx
+        ret
+@@:
         mov     eax, msg_board_data+1
         mov     ebx, msg_board_data
         movzx   edx, byte [ebx]
         call    memmove
         dec     [msg_board_count]
-        mov     [esp + SYSCALL_STACK.eax], edx ;eax
+        mov     [esp + SYSCALL_STACK.eax], edx
-        mov     [esp + 20], dword 1
+        mov     [esp + SYSCALL_STACK.ebx], 1
         ret
 
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