Homework 2 Getting Started with the LC-22

Homework 2 CS 2200 - Systems and Networks Spring 2022
1 Problem 1: Getting Started with the LC-22
In this homework, you will be using the LC-22 ISA to complete a Tower of Hanoi move-counting function.
Before you begin, you should familiarize yourself with the available instructions, the register conventions
and the calling convention of LC-22. Details can be found in the section, Appendix A: LC-22 Instruction
Set Architecture, at the end of this document.
The assembly folder contains several tools for you to use:
• assembler.py: a basic assembler that can take your assembly code and convert it into binary instructions for the LC-22.
• lc22.py: the ISA definition file for the assembler, which tells assembler.py the instructions supported
by the LC-22 and their formats.
• lc22-sim.py: A simulator of the LC-22 machine. The simulator reads binary instructions and emulates
the LC-22 machine, letting you single-step through instructions and check their results.
To learn how to run these tools, see the README.md file in the assembly directory.
Before you begin work on the second problem of the homework, try writing a simple program for the LC-22
architecture. This should help you familiarize yourself with the available instructions.
We have provided a template, mod.s, for you to use for this purpose. Try writing a program that performs
the mod operation on the two provided arguments. A correct implementation will result in a value of 2.
You can use the following C code snippet as a guide to implement this function:
int mod ( int a , int b ) {
int x = a ;
while ( x >= b ) {
x = x - b ;
}
return x ;
}
There is no turn-in for this portion of the assignment, but it is recommended that you attempt it in order
to familiarize yourself with the ISA.
Homework 2 CS 2200 - Systems and Networks Spring 2022
2 Problem 2: Tower of Hanoi
For this problem, you will be implementing the missing portions of the program that calculates the minimum
number of moves to solve the Tower of Hanoi problem for n disks.
Tower of Hanoi involves three vertical rods and a set of varying sized disks, which can slide onto any rod.
The disks are initially stacked on one of the rods in ascending order of size, with the largest disk on the
bottom and the smallest on top, thus making a conical shape. The objective of this puzzle is to migrate the
tower of disks completely to another rod, under the rule that only individual disks may be moved at once,
and no disks may be placed on smaller disks.
You will be finishing a recursive implementation of the Tower of Hanoi minimal moves calculator program
that follows the LC-22 calling convention. Recursive functions always obtain a return address through the
function call and return to the callee using the return address.
You must use the stack pointer ($sp) and frame pointer ($fp) registers as described in the
textbook and lecture slides.
Here is the C code for the Tower of Hanoi minimal moves calculator you have been provided:
int minimumHanoi ( int n ) {
if ( n == 1)
return 1;
else
return (2 * minimumHanoi ( n - 1) ) + 1;
}
Note that this C code is just to help your understanding and does not need to be exactly followed. However,
your assembly code implementation should meet all of the given conditions in the description.
Open hanoi.s file in the assembly directory. This file contains an implementation of the Tower of Hanoi
minimal moves calculator program that is missing significant portions of the calling convention. Near the
bottom of the hanoi.s we have provided multiple numbers that you can use to test your homework. They are
located at labels testNumDisks1, testNumDisks2, testNumDisks3. Be sure to use these provided integers
by loading them from the labels into registers. None of the numbers provided and tested will be lower than
1.
Complete the program by implementing the various missing portions of the LC-22 calling convention. Each
location where you need to implement a portion of the calling convention is marked with a TODO label as
well as a short hint describing the portion of the calling convention you should be implementing.
Please note that we will be testing your implementation for multiple different instances, so please do not
attempt to hardcode your solutions.
3 Problem 3: Short Answer
Please answer the following question in the file named answers.txt:
1. The LC-22 instruction set contains an instruction called jalr that is used to jump to a location while
saving a return address. However, this functionality could be emulated using a combination of other
instructions available in the ISA. Describe a sequence of other instructions in the LC-22 ISA that you
may use to accomplish the functionality of jalr.
For the purpose of this question, you may assume the target address is represented with the label
<target> which can be accessed using the 20 bits reserved for an offset or immediate value in the
LC-22 ISA.
Homework 2 CS 2200 - Systems and Networks Spring 2022
4 Deliverables
• hanoi.s: your assembly code from Section 2
• answers.txt: your answer to the problem from Section 3
Submit these files to Gradescope before the assignment deadline.
The TAs should be able to type python assembler.py -i lc22 --sym hanoi.s and then python lc22-sim.py
hanoi.bin to run your code. If you cannot do this with your submission, then you have done something
wrong.
Homework 2 CS 2200 - Systems and Networks Spring 2022
5 Appendix A: LC-22 Instruction Set Architecture
The LC-22 is a simple, yet capable computer architecture. The LC-22 combines attributes of both ARM
and the LC-2200 ISA defined in the Ramachandran & Leahy textbook for CS 2200.
The LC-22 is a word-addressable, 32-bit computer. All addresses refer to words, i.e. the first word
(four bytes) in memory occupies address 0x0, the second word, 0x1, etc.
All memory addresses are truncated to 16 bits on access, discarding the 16 most significant bits if the address
was stored in a 32-bit register. This provides roughly 64 KB of addressable memory.
5.1 Registers
The LC-22 has 16 general-purpose registers. While there are no hardware-enforced restraints on the uses of
these registers, your code is expected to follow the conventions outlined below.
Table 1: Registers and their Uses
Register Number Name Use Callee Save?
0 $zero Always Zero NA
1 $at Assembler/Target Address NA
2 $v0 Return Value No
3 $a0 Argument 1 No
4 $a1 Argument 2 No
5 $a2 Argument 3 No
6 $t0 Temporary Variable No
7 $t1 Temporary Variable No
8 $t2 Temporary Variable No
9 $s0 Saved Register Yes
10 $s1 Saved Register Yes
11 $s2 Saved Register Yes
12 $k0 Reserved for OS and Traps NA
13 $sp Stack Pointer No
14 $fp Frame Pointer Yes
15 $ra Return Address No
1. Register 0 is always read as zero. Any values written to it are discarded. Note: for the purposes of
this project, you must implement the zero register. Regardless of what is written to this register, it
should always output zero.
2. Register 1 is used to hold the target address of a jump. It may also be used by pseudo-instructions
generated by the assembler.
3. Register 2 is where you should store any returned value from a subroutine call.
4. Registers 3 - 5 are used to store function/subroutine arguments. Note: registers 2 through 8 should
be placed on the stack if the caller wants to retain those values. These registers are fair game for the
callee (subroutine) to trash.
5. Registers 6 - 8 are designated for temporary variables. The caller must save these registers if they
want these values to be retained.
6. Registers 9 - 11 are saved registers. The caller may assume that these registers are never tampered
with by the subroutine. If the subroutine needs these registers, then it should place them on the stack
and restore them before they jump back to the caller.
7. Register 12 is reserved for handling interrupts. While it should be implemented, it otherwise will not
have any special use on this assignment.
Homework 2 CS 2200 - Systems and Networks Spring 2022
8. Register 13 is the everchanging top of the stack; it keeps track of the top of the activation record for
a subroutine.
9. Register 14 is the anchor point of the activation frame. It is used to point to the first address on the
activation record for the currently executing process.
10. Register 15 is used to store the address a subroutine should return to when it is finished executing.
5.2 Instruction Overview
The LC-22 supports a variety of instruction forms, only a few of which we will use for this project. The
instructions we will implement in this project are summarized below.
Table 2: LC-22 Instruction Set
012345678910111213141516171819202122232425262728293031
ADD 0000 DR SR1 unused SR2
NAND 0001 DR SR1 unused SR2
ADDI 0010 DR SR1 immval20
LW 0011 DR BaseR offset20
SW 0100 SR BaseR offset20
BR 0101 unused offset20
JALR 0110 RA AT unused
HALT 0111 unused
BLT 1000 SR1 SR2 offset20
BGT 1001 SR1 SR2 offset20
LEA 1010 DR unused PCoffset20
5.2.1 Conditional Branching
Branching in the LC-22 ISA is a bit different than usual. We have a set of branching instructions including
BR, an unconditional branch, as well as BLT and BGT, which offer the ability to branch upon a certain
condition being met. The BLT and BGT instructions use comparison operators, comparing the values of
two source registers. If the comparisons are true (for example, with the BGT instruction, if SR1 > SR2),
then we will branch to the target destination of incrementedPC + offset20.
Note: The conditional branch instructions make use of the BrSel signal. Think about ways that you can use
this signal to implement conditional logic.
Homework 2 CS 2200 - Systems and Networks Spring 2022
5.3 Detailed Instruction Reference
5.3.1 ADD
Assembler Syntax
ADD DR, SR1, SR2
Encoding
012345678910111213141516171819202122232425262728293031
0000 DR SR1 unused SR2
Operation
DR = SR1 + SR2;
Description
The ADD instruction obtains the first source operand from the SR1 register. The second source operand is
obtained from the SR2 register. The second operand is added to the first source operand, and the result is
stored in DR.
5.3.2 NAND
Assembler Syntax
NAND DR, SR1, SR2
Encoding
012345678910111213141516171819202122232425262728293031
0001 DR SR1 unused SR2
Operation
DR = ~(SR1 & SR2);
Description
The NAND instruction performs a logical NAND (AND NOT) on the source operands obtained from SR1
and SR2. The result is stored in DR.
HINT: A logical NOT can be achieved by performing a NAND with both source operands the same.
For instance,
NAND DR, SR1, SR1
...achieves the following logical operation: DR ← SR1.
Homework 2 CS 2200 - Systems and Networks Spring 2022
5.3.3 ADDI
Assembler Syntax
ADDI DR, SR1, immval20
Encoding
012345678910111213141516171819202122232425262728293031
0010 DR SR1 immval20
Operation
DR = SR1 + SEXT(immval20);
Description
The ADDI instruction obtains the first source operand from the SR1 register. The second source operand is
obtained by sign-extending the immval20 field to 32 bits. The resulting operand is added to the first source
operand, and the result is stored in DR.
5.3.4 LW
Assembler Syntax
LW DR, offset20(BaseR)
Encoding
012345678910111213141516171819202122232425262728293031
0011 DR BaseR offset20
Operation
DR = MEM[BaseR + SEXT(offset20)];
Description
An address is computed by sign-extending bits [19:0] to 32 bits and then adding this result to the contents
of the register specified by bits [23:20]. The 32-bit word at this address is loaded into DR.
Homework 2 CS 2200 - Systems and Networks Spring 2022
5.3.5 SW
Assembler Syntax
SW SR, offset20(BaseR)
Encoding
012345678910111213141516171819202122232425262728293031
0100 SR BaseR offset20
Operation
MEM[BaseR + SEXT(offset20)] = SR;
Description
An address is computed by sign-extending bits [19:0] to 32 bits and then adding this result to the contents
of the register specified by bits [23:20]. The 32-bit word obtained from register SR is then stored at this
address.
5.3.6 BR
Assembler Syntax
BR offset20
Encoding
012345678910111213141516171819202122232425262728293031
0101 unused offset20
Operation
PC = incrementedPC + offset20
Description
A branch is unconditionally taken. The PC will be set to the sum of the incremented PC (since we have
already undergone fetch) and the sign-extended offset[19:0].
Homework 2 CS 2200 - Systems and Networks Spring 2022
5.3.7 JALR
Assembler Syntax
JALR RA, AT
Encoding
012345678910111213141516171819202122232425262728293031
0110 RA AT unused
Operation
RA = PC;
PC = AT;
Description
First, the incremented PC (address of the instruction + 1) is stored into register RA. Next, the PC is loaded
with the value of register AT, and the computer resumes execution at the new PC.
5.3.8 HALT
Assembler Syntax
HALT
Encoding
012345678910111213141516171819202122232425262728293031
0111 unused
Description
The machine is brought to a halt and executes no further instructions.
5.3.9 BLT
Assembler Syntax
BLT SR1, SR2, offset20
Encoding
012345678910111213141516171819202122232425262728293031
1000 SR1 SR2 offset20
Operation
if (SR1 < SR2) {
PC = incrementedPC + offset20
}
Description
A branch is taken if SR1 is less than SR2. If this is the case, the PC will be set to the sum of the incremented
PC (since we have already undergone fetch) and the sign-extended offset[19:0].
Homework 2 CS 2200 - Systems and Networks Spring 2022
5.3.10 BGT
Assembler Syntax
BGT SR1, SR2, offset20
Encoding
012345678910111213141516171819202122232425262728293031
1001 SR1 SR2 offset20
Operation
if (SR1 > SR2) {
PC = incrementedPC + offset20
}
Description
A branch is taken if SR1 is greater than SR2. If this is the case, the PC will be set to the sum of the
incremented PC (since we have already undergone fetch) and the sign-extended offset[19:0].
5.3.11 LEA
Assembler Syntax
LEA DR, label
Encoding
012345678910111213141516171819202122232425262728293031
1010 DR unused PCoffset20
Operation
DR = PC + SEXT(PCoffset20);
Description
An address is computed by sign-extending bits [19:0] to 32 bits and adding this result to the incremented
PC (address of instruction + 1). It then stores the computed address into register DR.