040B Read Function

This is Turbo Pascal's Read(File) function. It takes one parameter:

This reads bytes from File, if available.

SYS040B: MOV BX,SP

Use BX to address items on the stack.

Stack after SYS:040B

Index	Contents
BX	Return Address (OFFSET)
BX+02	Return Address (SEGMENT)
BX+04	Pointer to File/Text Record Data (OFFSET)
BX+06	Pointer to File/Text Record Data (SEGMENT)

SYS040D: PUSH DS

Save DS to the stack.

SYS040E: SS:
SYS040F: LES DI,[BX+04]

Loads the pointer to the File into ES:DI.

SYS0412: ES:
SYS0413: LDS DX,[DI:BufPtr]

Loads the pointer to buffer (BufPtr) in File into DS:DX.

SYS0416: ES:
SYS0417: MOV CX,[DI:BufSize]

Get size of the buffer (BufSize) in File into CX.

SYS041A: ES:
SYS041B: MOV BX,[DI:Handle]
SYS041D: MOV AH,3F
SYS041F: INT 21

Read from file or device using DOS INT 21h AH = 3Fh service with parameters:

SYS0421: JB 0433

Exit on error.

SYS0423: ES:
SYS0424: MOV [DI:BufEnd],AX

Set the actual number of bytes read on BufEnd in File.

SYS0427: XOR AX,AX
SYS0429: ES:
SYS042A: MOV WORD PTR [DI:BufPos],0000

Reset index (BufPos) of next character in buffer to 0 (beginning).

SYS042F: POP DS

Restore DS from the stack.

SYS0430: RETF 0004

Return to caller and pop-off parameters from the stack.

SYS0433: ES:
SYS0434: MOV WORD PTR [DI:BufEnd],0000
SYS0439: JMP 0429

On error, set the number of characters read (BufEnd) to 0.