diff --git a/ponydos.asm b/ponydos.asm
index 09647b6..269a168 100644
--- a/ponydos.asm
+++ b/ponydos.asm
@@ -15,12 +15,15 @@ FILE_MAX_SIZE equ 128
 jmp 0:start
 
 ; Thunks for application programs to use
-; 0x7c05
+; 0x7c05 SYS_OPEN_FILE
 call open_file
 retf
-; 0x7c09
+; 0x7c09 SYS_READ_SECTORS
 call read_sectors
 retf
+; 0x7c0d SYS_DRAW_RECT
+call draw_rect
+retf
 
 start:
 	cld
@@ -91,6 +94,7 @@ initialize_screen:
 	mov ch, 0x20
 	int 0x10
 
+draw:
 	; Set up segments for drawing routines
 	push word 0xb800
 	pop es
@@ -122,6 +126,61 @@ mainloop:
 ; Drawing subroutines
 ; ------------------------------------------------------------------
 
+; in:
+;  bx = width of input buffer
+;  cx = width of rectangle
+;  dx = height of rectangle
+;  ds:si = beginning of source data
+;  di = X
+;  bp = Y
+draw_rect:
+	pusha
+	push es
+
+	push word 0xb800
+	pop es
+
+	; Calculate the starting address in the screen buffer
+	; Assumes COLUMNS is 80
+	; X columns * 2 bytes / column
+	shl di, 1
+	; Y rows * 80 cells / row * 2 bytes / cell = Y * 160 bytes
+	; bp * 160 = bp * 128 + bp * 32 = (bp<<7) + (bp<<5)
+	shl bp, 5
+	add di, bp
+	shl bp, 2
+	add di, bp
+
+	; Convert widths to bytes
+	shl bx, 1
+	shl cx, 1
+
+	.loop:
+		; Any rows left to copy?
+		cmp dx, 0
+		je .end
+		dec dx
+
+		; Copy a row
+		push cx
+		rep movsb
+		pop cx
+
+		; Move to the next row in the input buffer
+		add si, bx
+		sub si, cx
+
+		; Move to the next row in the screen buffer
+		add di, 2*COLUMNS
+		sub di, cx
+
+		jmp .loop
+	.end:
+
+	pop es
+	popa
+	ret
+
 ; requires:
 ;  ds = 0
 ;  es = 0xb800
diff --git a/ponydos.inc b/ponydos.inc
index 5273276..86b71fd 100644
--- a/ponydos.inc
+++ b/ponydos.inc
@@ -2,6 +2,7 @@ PONYDOS_SEG equ 0
 
 SYS_OPEN_FILE equ 0x7c05
 SYS_READ_SECTORS equ 0x7c09
+SYS_DRAW_RECT equ 0x7c0d
 
 GLOBAL_WALLPAPER equ 0x500
 
diff --git a/shell.asm b/shell.asm
index ca0f379..a4c04e3 100644
--- a/shell.asm
+++ b/shell.asm
@@ -47,6 +47,27 @@ initialize:
 	mov bx, GLOBAL_WALLPAPER
 	call PONYDOS_SEG:SYS_READ_SECTORS
 
+	; Draw a rectangle on-screen
+	mov bx, 5
+	mov cx, 3
+	mov dx, 3
+	mov si, rect
+	mov di, 10
+	mov bp, 3
+	call PONYDOS_SEG:SYS_DRAW_RECT
+
+	call wait_key
 	ret
 
 wallpaper_name db 'wallpaper.bin', 0
+
+%include "debug.inc"
+
+rect:
+	db '0', 0xf0, '1', 0xf1, '2', 0xf2, '3', 0xf3, '4', 0xf4
+	db 'A', 0xf5,
+	times 4 db '*', 0xf0
+	db 'B', 0xf6
+	times 4 db '*', 0xf0
+	db 'C', 0xf7
+	times 4 db '*', 0xf0