INT 10h AH=01h

The text cursor is drawn by the card itself, as a blinking band of scan lines inside the character cell. Function 01h chooses the band: CH is its first line and CL its last, counted from 0 at the top of the cell. On a CGA a cell is 8 lines high and the usual underline is lines 6 to 7; monochrome and EGA cells are 14 lines high, VGA cells 16.

So that programs need not care, EGA and VGA BIOSes translate CGA-style values in the 25-line colour text modes, which IBM called cursor emulation: 0607h becomes the underline and 0007h a full block, whatever the real cell height. CGA values therefore work everywhere. Setting bit 5 of CH hides the cursor, which full-screen programs do while they draw.

The BIOS keeps the shape at 40h:60h in its data area, and function 03h returns it in CX. A program that changes it should read it first and put it back before it exits; DOS does not, and a missing or odd cursor at the prompt is confusing.

MOV     AH, 03h          ; read the cursor: CX = its shape
MOV     BH, 0
INT     10h
MOV     [SHAPE1], CX     ; keep it
MOV     AH, 01h          ; set the shape
MOV     CX, 0007h        ; scan lines 0 to 7: a full block
INT     10h
MOV     AH, 09h
MOV     DX, OFFSET MSG1  ; the block blinks at the end of this text
INT     21h
MOV     CX, 001Eh        ; wait 2 s: CX:DX = 1E8480h microseconds
MOV     DX, 8480h
MOV     AH, 86h          ; BIOS: wait
INT     15h
MOV     AH, 01h          ; the old shape back
MOV     CX, [SHAPE1]
INT     10h
MOV     AH, 09h
MOV     DX, OFFSET MSG2
INT     21h
MOV     AX, [SHAPE1]
CALL    PHEX4            ; the shape we found
MOV     AH, 09h
MOV     DX, OFFSET MSG3
INT     21h
 
SHAPE1  DW      0
MSG1    DB      "Block cursor for 2 seconds: $"
MSG2    DB      "back, it was $"
MSG3    DB      "h", 13, 10, "$"
PHEX4:                   ; print AX as four hex digits
PUSH    AX
MOV     AL, AH
CALL    PHEX2            ; the high byte first
POP     AX
PHEX2:                   ; print AL as two hex digits; changes AX, CX, DX
PUSH    AX
MOV     CL, 4
SHR     AL, CL           ; the high nibble first
CALL    PDIG1
POP     AX
AND     AL, 0Fh          ; then the low one, falling into PDIG1
PDIG1:
ADD     AL, "0"
CMP     AL, "9"
JBE     PDIG2
ADD     AL, 7            ; "A".."F"
PDIG2:
MOV     DL, AL
MOV     AH, 02h          ; DOS: print DL
INT     21h
RET

Pitfalls

There is a cursor only in text modes; in graphics modes the BIOS draws none and this function has no visible effect. On the original CGA, values above 7 or a first line below the last give a split or missing cursor; the VGA's emulation maps odd values in its own way, so keep to 0–7 in CGA terms. Emulation can be switched off with INT 10h AH=12h (BL = 34h), and then the values are real scan lines of the 16-line cell. A mode set restores the standard underline.