Description
In this lab, you will explore the more complex input/output capabilities of the DE1-SoC: the PS/2
keyboard port, and VGA display. You will begin by writing a driver for each of these devices, and
then put them together into an application.
Summary of Deliverables
● Source code for:
○ A library implementing a complete VGA driver
○ A library implementing a complete PS/2 driver
○ An application that draws maps on the VGA screen in response to PS/2 input
● Demo, no longer than five (5) minutes (week of April 4th)
● Report, no longer than four (4) pages (10 pt font, 1” margins) (due April 12th at 11:59
pm)
Please submit the above in a single .zip archive, using the following file name conventions:
● Code: part1.s, part2.s, part3.s
● Report: StudentID_FullName_Lab1_report.pdf
Grading Summary
● 50% Demo
● 50% Report
Changelog
● 21-Mar-2022 Initial revision
● 24-Mar-2022 Fixed a typo in Part 3 specifying what keys should be pressed to cycle
through flags.
Overview
In this lab we will use the high level I/O capabilities of the DE1-SoC simulator to
1. display pixels and characters using the VGA controller, and
2. accept keyboard input via the PS/2 port.
For each of these topics, we will create a driver. We will test the drivers both individually and in
tandem by means of test applications.
Part 1: Drawing things with VGA
The DE1-SoC computer has a built-in VGA controller that can render pixels, characters or a
combination of both. The authoritative resource on these matters is Sections 4.2.1 and 4.2.4 of
the DE1-SoC Computer Manual. This section of the lab provides a quick overview that should
suffice for the purpose of completing this lab.
To render pixels, the VGA controller continuously reads the pixel buffer, a region in memory
starting at 0xc8000000 that contains the color value of every pixel on the screen. Colors are
encoded as 16-bit integers that reserve 5 bits for the red channel, 6 bits for the green channel
and 5 bits for the blue channel. That is, every 16-bit color is encoded like so:
15 … 11 10 … 5 4 … 0
Red Green Blue
The pixel buffer is 320 pixels wide and 240 pixels high. Individual pixel colors can be accessed
at 0xc8000000 | (y << 10) | (x << 1), where x and y are valid x and y coordinates.
As previously hinted, we can also render characters. To do so, we will use the character buffer,
which is analogous to the pixel buffer, but for characters. The device’s VGA controller
continuously reads the character buffer and renders its contents as characters in a built-in font.
The character buffer itself is a buffer of byte-sized ASCII characters at 0xc9000000. The buffer
has a width of 80 characters and a height of 60 characters. An individual character can be
accessed at 0xc9000000 | (y << 7) | x.
Task: Create a VGA driver
To provide a slightly higher-level layer over the primitive functionality offered by the pixel and
character buffers, we will create a driver. That is, a set of functions that can be used to control
the screen.
To help get you started, we created an application that uses such functions to draw a testing
screen. Your job is to create a set of driver functions to support the application. Download vga.s
and augment it with the following four functions:
● VGA_draw_point_ASM draws a point on the screen with the color as indicated in the
third argument, by accessing only the pixel buffer memory. Hint: This subroutine should
only access the pixel buffer memory.
● VGA_clear_pixelbuff_ASM clears (sets to 0) all the valid memory locations in the
pixel buffer. It takes no arguments and returns nothing. Hint: You can implement this
function by calling VGA_draw_point_ASM with a color value of zero for every valid
location on the screen.
● VGA_write_char_ASM writes the ASCII code passed in the third argument (r2) to the
screen at the (x, y) coordinates given in the first two arguments (r0 and r1). Essentially,
the subroutine will store the value of the third argument at the address calculated with
the first two arguments. The subroutine should check that the coordinates supplied are
valid, i.e., x in [0, 79] and y in [0, 59]. Hint: This subroutine should only access the
character buffer memory.
● VGA_clear_charbuff_ASM clears (sets to 0) all the valid memory locations in the
character buffer. It takes no arguments and returns nothing. Hint: You can implement this
function by calling VGA_write_char_ASM with a character value of zero for every valid
location on the screen.
Their C prototypes are as follows:
void VGA_draw_point_ASM(int x, int y, short c);
void VGA_clear_pixelbuff_ASM();
void VGA_write_char_ASM(int x, int y, char c);
void VGA_clear_charbuff_ASM();
Notes:
● Use suffixes B and H with the assembly memory access instructions in order to
read/modify bytes/half-words in memory.
● You must follow the conventions taught in class. If you do not, then the testing code in
the next section will be unlikely to work.
Testing the VGA driver
To test your VGA driver, run your finished assembly file. You can inspect the VGA output visually
using the VGA pixel buffer tab under the Devices panel of the simulator.
If you implemented your driver correctly, compiling and running the program will draw the
following image.
Part 2. Reading keyboard input
For the purpose of this lab, here’s a high level description of the PS/2 keyboard protocol. For a
more comprehensive resource, see Section 4.5 (pp. 45-46) of the DE1-SoC Computer Manual.
The PS/2 bus provides data about keystroke events by sending hexadecimal numbers called
scan codes, which for this lab will vary from 1-3 bytes in length. When a key on the PS/2
keyboard is pressed, a unique scan code called the make code is sent, and when the key is
released, another scan code called the break code is sent. The scan code set used in this lab is
summarized by the table below. (Originally taken from Baruch Zoltan Francisc’s page on PS/2
scan codes.)
KEY MAKE BREAK KEY MAKE BREAK KEY MAKE BREAK
A 1C F0,1C 9 46 F0,46 [ 54 FO,54
B 32 F0,32 \` 0E F0,0E INSERT E0,70 E0,F0,7
0
C 21 F0,21 – 4E F0,4E HOME E0,6C E0,F0,6
C
D 23 F0,23 = 55 FO,55 PG UP E0,7D E0,F0,7
D
E 24 F0,24 \ 5D F0,5D DELETE E0,71 E0,F0,7
1
F 2B F0,2B BKSP 66 F0,66 END E0,69 E0,F0,6
9
G 34 F0,34 SPACE 29 F0,29 PG DN E0,7A E0,F0,7
A
H 33 F0,33 TAB 0D F0,0D U
ARROW E0,75 E0,F0,7
5
I 43 F0,43 CAPS 58 F0,58 L
ARROW E0,6B E0,F0,6
B
J 3B F0,3B L SHFT 12 FO,12 D
ARROW E0,72 E0,F0,7
2
K 42 F0,42 L CTRL 14 FO,14 R
ARROW E0,74 E0,F0,7
4
L 4B F0,4B L GUI E0,1F E0,F0,1
F NUM 77 F0,77
M 3A F0,3A L ALT 11 F0,11 KP / E0,4A E0,F0,4
A
N 31 F0,31 R SHFT 59 F0,59 KP * 7C F0,7C
O 44 F0,44 R CTRL E0,14 E0,F0,1
4 KP – 7B F0,7B
P 4D F0,4D R GUI E0,27 E0,F0,2
7 KP + 79 F0,79
Q 15 F0,15 R ALT E0,11 E0,F0,1
1 KP EN E0,5A E0,F0,5
A
R 2D F0,2D APPS E0,2F E0,F0,2
F KP . 71 F0,71
S 1B F0,1B ENTER 5A F0,5A KP 0 70 F0,70
T 2C F0,2C ESC 76 F0,76 KP 1 69 F0,69
U 3C F0,3C F1 05 F0,05 KP 2 72 F0,72
V 2A F0,2A F2 06 F0,06 KP 3 7A F0,7A
W 1D F0,1D F3 04 F0,04 KP 4 6B F0,6B
X 22 F0,22 F4 0C F0,0C KP 5 73 F0,73
Y 35 F0,35 F5 03 F0,03 KP 6 74 F0,74
Z 1A F0,1A F6 0B F0,0B KP 7 6C F0,6C
0 45 F0,45 F7 83 F0,83 KP 8 75 F0,75
1 16 F0,16 F8 0A F0,0A KP 9 7D F0,7D
2 1E F0,1E F9 01 F0,01 ] 5B F0,5B
3 26 F0,26 F10 09 F0,09 ; 4C F0,4C
4 25 F0,25 F11 78 F0,78 ‘ 52 F0,52
5 2E F0,2E F12 07 F0,07 , 41 F0,41
6 36 F0,36 PRNT
SCRN
E0,12,
E0,7C
E0,F0,
7C,E0,
F0,12
. 49 F0,49
7 3D F0,3D SCROL
L 7E F0,7E / 4A F0,4A
8 3E F0,3E PAUSE
E1,14,7
7,
E1,F0,1
4,
F0,77
Two other parameters involved are the typematic delay and the typematic rate. When a key is
pressed, the corresponding make code is sent, and if the key is held down, the same make code
is repeatedly sent at a constant rate after an initial delay. The initial delay ensures that briefly
pressing a key will not register as more than one keystroke. The make code will stop being sent
only if the key is released or another key is pressed. The initial delay between the first and
second make code is called the typematic delay, and the rate at which the make code is sent
after this is called the typematic rate. The typematic delay can range from 0.25 seconds to 1.00
second and the typematic rate can range from 2.0 cps (characters per second) to 30.0 cps, with
default values of 500 ms and 10.9 cps respectively.
Task: Create a PS/2 driver
The DE1-SoC receives keyboard input from a memory-mapped PS/2 data register at address
0xff200100. Said register has an RVALID bit that states whether or not the current contents of
the register represent a new value from the keyboard. The RVALID bit can be accessed by
shifting the data register 15 bits to the right and extracting the lowest bit, i.e.,
RVALID = ((*(volatile int *)0xff200100) >> 15) & 0x1. When RVALID is true,
the low eight bits of the PS/2 data register correspond to a byte of keyboard data.
The hardware knows when you read a value from the memory-mapped PS/2 data register and
will automatically present the next code when you read the data register again.
For more details, see Section 4.5 (pp. 45-46) of the DE1-SoC Computer Manual.
Download ps2.s. This assembly file implements a program that reads keystrokes from the
keyboard and writes the PS/2 codes to the VGA screen using the character buffer. Copy your
VGA driver into ps2.s. Then implement a function that adheres to the following specifications:
● Name: read_PS2_data_ASM
● Input argument (r0): A memory address in which the data that is read from the PS/2
keyboard will be stored (pointer argument).
● Output argument (r0): Integer that denotes whether the data read is valid or not.
● Description: The subroutine will check the RVALID bit in the PS/2 Data register. If it is
valid, then the data from the same register should be stored at the address in the pointer
argument, and the subroutine should return 1 to denote valid data. If the RVALID bit is
not set, then the subroutine should simply return 0.
read_PS2_data_ASM’s C declaration is as follows:
int read_PS2_data_ASM(char *data);
Testing the PS/2 driver
To verify that the PS/2 driver is working correctly, you can type into the simulator’s PS/2
keyboard device and verify that the bytes showing up on the screen correspond to the codes
you might expect from the table in this section’s introduction.
If you implemented your PS/2 and VGA drivers correctly, then the program will print make and
break codes whenever you type in the simulator’s keyboard input device. Make sure to use the
keyboard device that says ff200100.
Note: If you did not manage to implement a working VGA driver, then you can still get credit for
the PS/2 driver by replacing write_byte with the implementation below. It will write PS/2
codes to memory address 0xfff0. Delete all calls to VGA driver functions and delete the
write_hex_digit function to ensure that your code still compiles.
write_byte:
push {r3, r4, lr}
ldr r4, =0xfff0
and r3, r3, #0xff
str r3, [r4]
pop {r3, r4, pc}
Part 3. Putting everything together: Vexillology
We will now create an application that paints a gallery of flags. The user can use keyboard keys
to navigate through flags. Pressing the D key will prompt the application to show the next flag.
Similarly, pressing the A key will prompt the application to show the previous flag. Pressing A
when at the first flag or D when at the last flag cycles to the last or first flag, respectively.
Download flags.s. This file implements an input loop that reads keystrokes from the keyboard,
tests if they are A or D key presses, and cycles flags accordingly. It also implements a function
that draws the flag of Texas.
Your task is twofold:
1. Include your VGA and PS/2 driver functions.
2. Implement flag painting logic for two other flags by rewriting draw_real_life_flag
and draw_imaginary_flag . One flag must be a real-life flag and another flag can be
a flag you designed yourself.
Hint: The starter code includes the draw_rectangle and draw_star functions. You can use
these functions to draw any flag that consists of rectangles and stars. This encompasses many
flags, from the flags of the US to France and even China.
draw_rectangle draws a rectangle. It takes five arguments. The first four arguments are
stored in registers r0 through r3. The fifth argument is stored on the stack at address [sp].
draw_rectangle’s signature is:
/**
* Draws a rectangle.
* @param x The x coordinate of the top left corner of the rectangle.
* @param y The y coordinate of the top left corner of the rectangle.
* @param width The width of the rectangle.
* @param height The height of the rectangle.
* @param c The color with which to fill the rectangle.
*/
void draw_rectangle(int x, int y, int width, int height, int c);
draw_star draws a star. It takes four arguments, stored in registers r0 through r3. Its signature is:
/**
* Draws a star.
* @param x_center The x coordinate at which the star is centered.
* @param y_center The y coordinate at which the star is centered.
* @param radius The star’s radius.
* @param c The star’s color.
*/
void draw_star(int x_center, int y_center, int radius, int c);
Hint: The flag of Texas as drawn by the starter code should look like the image below. If it
doesn’t, then your VGA driver might be faulty.
Deliverables
Your demo is limited to 5 minutes. It is useful to highlight that your software computes correct
partial and final answers; draw our attention to the registers and memory contents at appropriate
points to demonstrate that your software operates as expected.
Your demo will be graded by assessing, for each software deliverable, the correctness of the
observed behavior, and the correctness of your description of that behavior.
In your report, for each software deliverable, describe:
● Describe your approach (e.g., how you used subroutines, the stack, etc)
● Challenges you faced, if any, and your solutions
● Shortcomings, possible improvements, etc
Note that you need not describe the operation of the given libraries, only how you made use of
the provided functions.
Your report is limited to four pages, total (no smaller than 10 pt font, no narrower than 1”
margins). It will be graded by assessing, for each software deliverable, your report’s clarity,
organization, and technical content.
Grading
Your demo and report are equally weighted. The breakdown for the demo and report are as
follows:
Demo
● 30% Part 1: VGA driver
● 30% Part 2: PS/2 driver
● 40% Part 3: Vexillology application
Each section will be graded for (a) clarity, (b) technical content, and (c) correct execution:
● 1pt clarity: the demo is clear and easy to follow
● 1pt technical content: correct terms are used to describe your software
● 3pt correctness: given an input, the correct output is clearly demonstrated
Report
● 30% Part 1: VGA driver
● 30% Part 2: PS/2 driver
● 40% Part 3: Vexillology application
Each section will be graded for: (a) clarity, (b) organization, and (c) technical content:
● 1pt clarity: grammar, syntax, word choice
● 1pt organization: clear narrative flow from problem description, approach, testing,
challenges, etc.
● 3pt technical content: appropriate use of terms, description of proposed
approach, description of testing and results, etc.
Submission
Please submit, on MyCourses, your source code and report in a single .zip archive, using the
following file name conventions:
● Code: part1.s, part2.s, part3.s
● Report: StudentID_FullName_Lab1_report.pdf