INT 21h - Exec function: Out Of Memory

[Peter Swinkels]

EDIT:
I solved the problem with INT 21h, 48h (Allocate Memory Blocks)
I should write MOV ah, 48h, not MOV ax, 48h
That only leaves the problem with Exec function to be solved...

I am trying to write a routine that can run other programs but whenever I run this program INT 21h, 4Bh (Exec Load and Execute Program) reports that the program is out of memory. (error code 8) As far as I understand it I need to allocate memory for another program before running it. I can't find any useful information in Helppc or on the internet though. I tried allocating memory with INT 21h, 48h but the program exits instead of allocating memory whenever it attempts to execute that function.

The program: (All numbers are in DECIMAL notation! Which means that INT 21h, 4Bh is referred to as INT 33, 75 in the code.)

ORG 256
Main:
	; Executes the program.
	MOV ah, 75		; Load and Execute Program
	MOV al, 0		; Load and execute.
	MOV dx, .Program	; The path and file name of the program to execute.
	MOV bx, ds		; Parameter block.
	MOV es, bx		;
	MOV bx, .Parameters	;
	INT 33			; DOS functions interrupt.
	JMP ErrorTrap
RET

.Parameters RESB 22
.Program db "command.com", 0



;+---------------------------------------------------+
;| Error Trap - Displays the error code. 	     |
;| [Parameters in:]			 	     |
;| Name:	Description:		Type:	     |
;| ax		The error code.		CPU register |
;+---------------------------------------------------+

ErrorTrap:
	; Converts the error code to a user readable format.
	PUSH ax			; The error code.
	CALL cv256to16		; Converts value to user readable format.
	POP DWORD [.ErrorCode]	;

	; Displays the error code.
	MOV ah, 9		; Print String.
	MOV dx, .ErrorCode	;
	INT 33			; DOS functions interrupt.
RET

.ErrorCode db "0000", 13, 10, "$"



;+-------------------------------------------------------------------------------+
;| Convert Byte to Hexadecimal - Converts byte values to hexadecimal number.     |
;| [Parameters in:]                                                 		 |
;| Name:	Description:				Type:    		 |
;| Number	Byte values to be converted.            2 Byte number (by value) |
;| Parameters out:]                                                     	 |
;| Name:	Description:				Type:    		 |
;| Hexadecimal	Converted hexadecimal number.		4 Byte string (by value) |
;+-------------------------------------------------------------------------------+

cv256to16:
	POP WORD [.ReturnAddress]
	POP WORD [.ByteVal]

	MOV bx, 0					; Digit = 0
	MOV cl, 12					; BitShift = 12
	MOV dx, 64410					; BitMask = 64410
		.GetHexDigits:
			MOV ax, [.ByteVal]		;
			AND ax, dx			; Extract a 4 bit value from the byte values.
			SHR ax, cl			;
			MOV si, ax			;
			MOV al, [.HexDigits + si]	; Get the hexadecimal digit for the 4 bit value.
			MOV [.HexNumber + bx], al	;
			SHR dx, 4			; Adjust BitMask
			SUB cl, 4			; and BitShift.
			INC bx				; Next digit.
			CMP bx, 4			; Check for last digit.
		JNE .GetHexDigits

	PUSH DWORD [.HexNumber]
	PUSH WORD [.ReturnAddress]
RET

.ByteVal dw 0
.HexNumber db "0000"
.HexDigits db "0123456789ABCDEF"
.ReturnAddress dw 0