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SC123 Assembly Language Manual
SC123 Website: http://www.cs.csustan.edu/~rrsilver/html/sc123.html
Dr. Robert Silverman
Computer Science Department
California State University Stanislaus
Turlock, CA 95382
rsilverman@csustan.edu
Created date: May 1, 2006
Revised date: April 29, 2007
Revised date: June 8, 2007
Revised date: June 16, 2007
Revised date: July 6, 2007
Revised date: July 20, 2007
Revised date: August 27, 2007
Revised date: November 9, 2007
Revised date: January 7, 2008
Revised date: October 13, 2008
Revised date: November 20, 2008
Revised date: May 11, 2009
Revised date: November 17, 2009
Revised date: July 30, 2010
Revised date: October 5, 2010
Revised date: October 25, 2010
Revised date: October 7, 2011
Revised date: February 5, 2012
Revised date: August 26, 2012
Revised date: October 2, 2012
Revised date: October 15, 2012
Revised date: January 20, 2013
Revised date: July 20, 2014
Note: Key changes at architecture level in January 2012: User space allocation. Stack behavior. Old programs
using stack will not work and require user to read this document and then to modify their programs.
This document and system is copyrighted by Robert R. Silverman, 2002, ... current date. No part of this
system or information presented here can be reproduced using any means, such as: book print, physical,
electronic, chemical, biological, mathematical, telepathic, or other not yet discovered means without the
explicit permission by Dr. Robert R. Silverman, CSU Stanislaus, rsilverman@csustan.edu An explicit
permission is given to all my students to copy this material as needed for educational noncommercial
purpose.
This document can be obtained for free from Dr. Silverman for educational noncommercial purpose.
Robert Silverman
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TOC
SC123 Assembly Language Manual.........................................................................................................1
Introduction...........................................................................................................................................3
Overview of SC123 System..................................................................................................................3
System Organization Diagrams............................................................................................................4
Instruction Set.......................................................................................................................................5
Address Space Partition........................................................................................................................5
Registers and Memory..........................................................................................................................5
Addressing Modes................................................................................................................................5
Operand Addressing Mode Usage........................................................................................................6
Machine Instruction Format..................................................................................................................7
Instruction Semantics............................................................................................................................7
Basic Instructions (group 0)..............................................................................................................8
Logic and Shift Instructions (group 1)..............................................................................................9
Input/output Instructions (group 2)...................................................................................................9
Jump Instructions (group 3)...............................................................................................................9
Subroutine Call Instructions (group 4)............................................................................................11
Data Definition....................................................................................................................................11
Data Semantics....................................................................................................................................11
I/O Device Programming....................................................................................................................11
Example of a Simple Program............................................................................................................12
Program Example...............................................................................................................................13
Programming Fragments Examples....................................................................................................14
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Introduction
This is the Assembly Language Manual for SC123, a new Educational computer Developed at CSU Stanislaus.
This manual has four main parts:
● System overview
● SC123 organization overview
● Assembly language syntax and semantics.
● Code tutorial
The SC123 is a new computer architecture. This manual is for the ID3E programming and execution
environment. The ID3E consists of an Assembler, IDE, and an Execution engine all packaged in a friendly GUI
accessible system.
The ID3E provides assembly language programming teaching/learning environment that will:
1. Provide instructors with the capability of staging the learning process of the instruction architecture
from basic operations to more complex operations to a complete architecture.
2. Provide instructors with a convenient tool within which to test students programs.
3. Provide students with a realistic assembly language programming experience that is readily transferable
to any industry setting in which they would need to use assembly language.
4. Provide students with a convenient tool to learn, write, test, debug, and execute their programs.
Overview of SC123 System
The machine has a simple architecture to make the language easy to learn. Here is the registers view of the
architecture. The functional view is discussed in the ISA section. The machine contains the following user
program accessible registers. Register SP is mapped into r3. We will refer to it as SP or r3 as appropriate.
Register
name
functionality
PC
program counter 16 bits
holds the address of the current instruction
IR
instruction register 48 bits
holds instruction including two addresses (if
needed)
r0-r3
general purpose reg 16 bits register to hold operands for computation or
address of operands
PSR
processor status rg 16 bits
currently contains only CCstate mapped into the
five least significant bits of PSR
CCstate
Condition code state
negative (N), positive (P), Zero (Z), carry (C), and
overflow (O)
SP/r3
stack pointer/r3 16 bits
points to top of stack (empty slot)
stack grows down. In other words, pushing a word
onto stack cause sp to decrement. The semantic
was changed in January 2012.
inport
input register 16 bits
brings data from input devices
outport
output register 16 bits
sends data to output devices
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CCstate[N], CCstate[Z], CCstate[P], CCstate[C], and CCstate[O] are condition code bits which are affected by
arithmetic operations. They are used for conditional jump (jumpct, jumpcf), described in jump section.
The valid address space is 2^15 words; hence the valid address range is 0x0000-0x7FFF. If you want to know
why the valid space is not 2 ^ 16 words, see how real computers deal with negative address.
System Organization Diagrams
The following is the SC123 organization diagram. The organization is simple and can be used in the first course
on assembly language, course on computer architecture, or a course on computer organization. For clarity,
omitted from the diagram is PSR and its logic. PSR.CCstate is affected by results of arithmetic operations. We
will call PSR.CCstate the “condition code state”.
Figure: SC123 Organization Diagram
Main Memory
CPU
regs
IR-Dec
R3-Inc
PC-Inc
MAR
ALU
MM
MDR
Data bus
Address bus
Control bus
SEQ
Inport
Outport
Input/Output
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Instruction Set
For pedagogical purposes, the SC123 instruction set is divided into a number of disjoint op-code groups. This
allows the instructor to teach the instructions in logically related groups.
It is tempting to describe these instruction set divisions as hierarchical, adding layers of sophistication.
However, this would be misleading. Different teachers might present concepts in a different order, so the
instruction set might grow perfectly logically in more than one direction. While one instructor might want to
teach arithmetical operations first, followed by logical operations, and then subroutines, another might leave
logical operations out of the course entirely, but introduce indirect addressing.
In addition to the partitioned op-code set, the SC123 uses an orthogonal set of addressing modes which are
available to most instructions. Addressing modes which are orthogonal to the op-code set are a feature of most
(though not all) modern computer architectures. They simplify the instruction set architecture while enhancing
the computational power and efficiency. Pedagogically, orthogonality allows the two concepts -- operations and
addressing -- to be taught separately.
Address Space Partition
The computer address space is partitioned into three subspaces.
[0x00-0x0F] Interrupt space
[0x10-0x3F] I/O space
[0x40-(MMsize-1)] user program space.
Registers and Memory
The SC123 CPU includes four general purpose registers, stack pointer (mapped to r3), a program counter, and a
processor status register which contains the condition code register. Each of these, except the condition code
register, is sixteen bits in size.
The condition code register (CCstate) (condition code state) has separate bits for "negative," "zero," "positive,"
"carry," and "overflow" conditions:
Name
Meaning
Negative
Zero
Positive
Carry
Overflow
Operation result is less than zero
Operation result is zero
Operation result is greater than zero
Operation resulted in an arithmetic carry
Operation result is larger than will fit in
destination
CCstate Register
bit
0
1
2
3
4
Memory consists of sixteen-bit words, addressable on a word basis. Memory is used to store data, program and
stack.
Addressing Modes
The bulk of the SC123 instruction set uses two operands, designated as the source and destination operands,
with the following "basic addressing modes". If information is in main memory, we will use concept of effective
address (ea), to describe how the data is accessed. The data is accessed in main memory using the ea, but the ea
is computed differently in different modes.
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Register: This addressing mode specifies one of the four general purposes registers. If src is register, the
information is in register. If destination is register, then the data will be deposited in the register.
Memory: This mode indicates a location in main memory. When the memory addressing mode is used, the
instruction is followed by a sixteen-bit fixed address. Note that although the machine-level architecture has a
single memory addressing mode, the assembler makes distinctions between general memory addresses, data
variables, and program labels. If operand uses memory mode, then ea = address. Variable names must be at least
2 chars long.
Literal: This addressing mode specifies the data as a part of the instruction, rather than indicating where to find
it. When it is used, the instruction is followed by a sixteen-bit fixed data value. This addressing mode is only
permitted with source operands, for obvious reasons. If source uses literal mode, then the information is in the
instruction.
Register Indirect with Offset: This addressing mode specifies a register that contains a base address, plus an
offset to be added to that base address to produce the memory address of the operand. When used, the
instruction is followed by a sixteen-bit fixed offset. Register indirect with offset addressing can be used in a
wide range of applications, including register indirect addressing (with a zero offset), stack operations (using a
register as the stack pointer, and incrementing or decrementing the register using the add and subtract
instructions), and record references (with the base address of the record in the register and the in-record offset of
the data in the offset). For stack operations, r3 is used as the stack pointer by convention. The effective address
is computed as follows: ea = r + offset. The data is contained in MM[ea]. The offset can be positive or negative.
Control flow instructions (jump, conditional jumpct, jumpcf, and jump to subroutine) use the "jump addressing
mode" for its address.
Jump addressing mode: This addressing mode specifies a memory location, and is used exclusively as the target
of a control flow jump. When it is used, the instruction is followed by a sixteen-bit fixed address. Although this
mode is similar to the previously described memory mode, we consider it a separate mode for the sake of clarity,
since it references a memory location rather than its contents.
Operand Addressing Mode Usage
Some instructions have no operands, such as nop and halt. Some instructions have operands and need to use
addressing modes to access the operands. The addressing modes are as general as possible, but clearly some
addressing modes cannot be applied in certain situations. For example, literal cannot be a destination. NA
means- not applicable.
instruction
Op1 permitted modes
Op2 permitted modes
nop, halt, clearcc
NA
NA
move, add, sub, and, nor, shl, shr
rg, mm, lit, rg(off)
rg, mm, rg(off)
in, pop
rg, mm, rg(off)
NA
out, push
rg, mm, lit, rg(off)
NA
jumpa
jump-mode
NA
jumpct, jumpcf
rg, mm, lit, rg(off)
jump-mode
jsr
jump-mode
NA
ret
NA
NA
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Machine Instruction Format
SC123 instructions are divided into four nibbles, (a nibble is four bits): the op-code group, the op-code, the
source operand, and the destination operand.
For pedagogical reasons, the instruction set consists of several instruction groups. We have the following
instruction groups, the basic group (G0), logical and shift group (G1), input/output group (G2), control flow
group (G3), and subroutine call group (G4). Based on our experience in the classroom, we may decide to add
additional groups.
Group
Op-code
Source
Destination
The group number and op-code fields are simple, four-bit numbers. The source and destination fields contain
addressing modes.
For the four basic addressing modes and jump mode consist of two, two-bit sub-fields: the select mode bits and
a select register bits.
Address mode:
select mode bits
select register bits
The address mode field can have the following values:
Address Mode
Register
Memory
Literal (source operands only)
Register indirect with offset
jump
select mode bits
00
01
10
11
01
select register bits
00-11
00
00
00-11
01
Note that the memory, literal, and register indirect with offset address modes involve an additional word
appended to the instruction to hold the memory location or literal value.
The control flow instructions have only one mode for its address, the jump address mode.
The jump address mode:
The jump address mode is specified by 0101[2].
Instruction Semantics
In the following sections, we describe the semantics of each of the instructions. The following abbreviations are
used:
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PSR: processor status register (currently containing only CCstate)
PC: program counter
r0-r3: the four general purpose registers
il: instruction length
CCstate: condition code register (aka “condition code state”) Note that CCstate is mapped into the five least
significant bits of PSR.
N: negative condition code
Z: zero condition code
P: positive condition code
C: carry condition code
O: overflow condition code
src: source operand (see addressing mode discussion above)
dst: destination operand (see address mode discussion above)
addr: instruction address operand (see address mode discussion above)
Most of these are obvious, but the instruction length, il, requires some additional explanation. SC123
instructions are one, two, or three words in length, depending upon the addressing modes used. The basic
instruction size is one word. A source operand of memory, literal, or register indirect with offset type adds an
additional word (to hold the memory address or literal value). A destination operand of memory type (literal
being illegal for destination operands) adds an additional word (to hold the memory address). The il is the total
length of the current instruction.
Some instructions affect no condition code state, some affect some bits of condition code state, and some
instructions affect all bits of condition code state. If an instruction does not affect a bit, the bit stays unchanged.
Basic Instructions (group 0)
The basic instruction set includes the most basic operations available in the SC123.
Operation
Description
No-op
Halt
Clear
CCstate
Move
No operation
Halt computation
Clear condition codes
Move word from src to dst move
3
Add
Add src to dst
add
4
Subtract
Subtract src from dst
sub
5
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Mnemonic Opcode
nop
0
halt
1
clearcc
2
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Semantics
PC + il -> PC
0 -> CCstate
PC + il -> PC
src -> dst
PC + il -> PC
src + dst -> dst
PC + il -> PC
dst - src -> dst
PC + il -> PC
CCstate
affected
None
None
All
None
All
All
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Logic and Shift Instructions (group 1)
The logic instructions include basic bit-wise manipulations. This is logical shift by number of bits specified by
cnt. For shl, bits shifted to the left are lost. We inject zeroes from right. For shr, bits shifted to the right are lost
and zeroes are injected from left.
Operation
Description
And
Bit-wise logical and
Mnemonic Opcode
and
0
Nor
Bit-wise logical nor
nor
1
Shift left
Bit-wise logical left shift
shl
2
Shift right
Bit-wise logical right shift shr
3
Semantics
CCstate
affected
N, Z, P
src AND dst -> dst;
PC + il -> PC;
src NOR dst -> dst;
N, Z, P
PC + il -> PC;
src -> cnt;
N, Z, P
dst shifted left by cnt
-> dst;
PC + il -> PC;
src -> cnt;
N, Z, P
dst shifted right by cnt
-> dst;
PC + il -> PC;
Shift left operation:
Ex: shl #2 val 0100 0000 1000 1100
Result: 0000 0010 0011 0000.
In other words:
If (i + cnt) < 16, shift bi left by count cnt.
If (i + cnt) >= 16, discard bi.
Inject 0 from right.
Input/output Instructions (group 2)
The input/output instructions allow the SC123 to communicate with the outside world via a single input and a
single output channel.
Operation
Description
In
Input data from external
device
Output data to external
device
Out
Mnemonic Opcode
in
0
out
1
Semantics
inport -> dst
PC + il -> PC
src -> outport
PC + il -> PC
CCstate
affected
none
none
Jump Instructions (group 3)
We have unconditional jumpa (always). Conditional jumps provide for condition-based change of control flow.
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Jumpct jumps if condition is true. jumpcf jumps if condition is false. A single conditional jump instruction can
test any combination of the processor's condition code state (CCstate).
Description of jumpct:
jumpct <CCcond> <address>. The jump condition "CCcond" can specify a logical "or" of multiple primitive
conditions. The primitive conditions are the following: N, Z, P, C, and O.
We can jump for example, on “P or Z”.
The PSR stores five condition code state (CCstate) bits as five bits, where the least significant is b0. The bits are
b4= [O], b3=[C], b2= [P], b1= [Z], and b0= [N].
To determine if jumpct will take the jump, we need to compute the boolean condTotal. The system hardware
evaluates the condTotal and if it is true, then the jump is taken. Otherwise, jump is not taken. There are two
components to compute condTotal; we need to know CCstate and CCcond.
The CCcond specified in the jumpct can be a number between 0 and 31 (base 10). This is equivalent to 00000 to
11111 (base 2). Higher bits than b4, will be ignored.
The CCcond is specified in decimal, but can be converted to 5-bit binary number.
For example, if CCcond=#3 (base 10), that is equivalent to 00011 (base 2). If CCcond=#1 (base 10), that is
equivalent to 00001 (base 2).
The condTotal is computed as follows.
condTotal = ORi=0 i=4 (CCstate[i] AND CCcond[i]]); the "logical or" is taken from i = 0 to 4.
In other words: condTotal = (CCcond[4] AND CCstate[4]) OR (CCcond[3] AND CCstate[3]) OR (CCcond[2]
AND CCstate[2]) OR (CCcond[1] AND CCstate[1]) OR (CCcond[0] AND CCstate[0])
For example:
jumpct #1 200, #1 (base 10) = 00001 (base 2), will take the jump if CCstate[0] ==1, in other words, jump on
negative.
jumpct #3 200, #3 (base 10) = 00011 (base 2), will take the jump if CCstate[1] ==1 or CCstate[0] ==1, in other
words, jump on zero or negative.
Jumpct #16 200, will jump if CCstate[4] ==1, in other words, jump on overflow.
jumpcf behaves similarly, except it jumps if the condTotal evaluates to false.
Operation
Description
Unconditiona Jump always
l jump
Conditional Jump based on condition
jump if
code values and will be
condTotal is taken if condTotal = true
true
Conditional Jump based on condition
jump if
code values and will be
condTotal is taken if condTotal = false
false
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Mnemonic Opcode
jumpa
0
Semantics
jumpct
1
if (condTotal = true)
then addr -> PC
else PC + il -> PC
None
jumpcf
2
if (condTotal =false)
then addr -> PC
else PC + il -> PC
None
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addr -> PC
CCstate
affected
None
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Subroutine Call Instructions (group 4)
These instructions allow subroutines to be called, and data to be pushed onto and popped off of the stack. Regis ter r3 is used as the stack pointer. The semantic was changed in January 2012.
Operation
Description
Semantics
Return
Mnemonic Opcode
Call a subroutine, pushing jsr
0
a return
address on the stack
Return from a subroutine ret
1
Push
Push data onto the stack
push
2
Pop
Pop data from the stack
pop
3
PC + il -> M[SP]
SP - 1 -> SP
addr -> PC
SP + 1 -> SP
M[SP] -> PC
src -> M[SP]
SP - 1 -> SP
PC + il -> PC
SP + 1 -> SP
M[SP] -> dst
PC + il -> PC
Jump to
subroutine
CCstate
affected
None
None
None
None
Data Definition
Data size is one word. We can define uninitialized data like so:
cnt
word
//reserve 1 word uninitialized
array word 10
//reserve 10 word uninitialized
sum
initword
#13
//reserve 1 word initialized to 13
Data Semantics
We have one data type: 16 bit integer in 2s complement notation.
All arithmetic uses 2s complement rules.
I/O Device Programming
The simulator models I/O subsystem. The I/O space is memory mapped. Currently implemented I/O device is
Printer. In the future we may add hard disk. Each I/O device is represented by interface as follows.
Name
DevTypeRg
CtrRg
StatRg
DataRg
Usage
Register contains the type of I/O device (read only)
Hold commands (write only)
Contains status of the device (read only)
Data to/from device (read/write)
Rg offset within interface
0
1
2
3
Device
Interrupts
Address allocated
0x00-0x0F
State of affairs
done
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Printer
Disk
Reserved
0x10-0x1F
0x20-0x2F
0x30-0x3F
done
future
future
Printer status register bits mapping. The status register has the following assignment.
Bit
0
1
2
3
4
5
6
Bit name
bitgoodproto
bitinproto
bitrdy
bit3
bitoper
bitjam
bit6
function 1/0
the device is within good protocol/ otherwise bad protocol
the device is within protocol/ not in protocol
DataRg is ready for new data/ not ready
NU
the device is operational/ the device is not operational
the device has soft error/ the device does not have soft error
NU
Devices commands (new commands may be added later). Also listing special control data.
Value
0xF001
0xF002
0xF003
0xF0040xF00F
command
PROTOSTART
PROTOEND
PROTORESET
reserved
function
start protocol
end protocol
reset protocol
Value
control data
function
0x8000
NEWLINE
tells printer to go to new line
For sample printer program printing a char on the printer see the program in WrkSpace dir.
Example of a Simple Program
Here is an example program in SC123 assembly language intentionally using a broad range of instructions and
different addressing modes. In this case we will take an array and modify each element by adding 3 to it. Note
that each program has four required blocks, the comment header, data, text, and stack. Each program block is
delimited by <X>Start and <X>End assembly directives. Omission of any of them will generate an assembly
error.
The data, text, and stack blocks directives include user specified start address where the block will be located in
main memory.
For example,
!textStart
90
means that the text block starts in location 90[10] in MM. This teaches students that programs take space.
The program can use decimal and hexadecimal numbers for values and addresses. Hexadecimal number is
represented by 0xNNNN, where N is hex digit, 0...F.
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Program Example
!commentStart
//The slash-slash denotes that this line is a comment.
//Comment must be alone on its own line.
//Line that starts with ! is an assembler directive.
//Space between two fields can be single or multiple blank(s) or tab(s).
//Every program consists of four adjacent blocks: comment, data, text and stack.
//There can be blank line anywhere, except between adjacent blocks.
//This program goes through an array and adds 3 to each element.
//This is not written the most efficient way – it is this way – like everything else – for pedagogical reason.
!commentEnd
!dataStart
0x80
//array data start
myArray
initword
#11
initword
#12
initword
#13
initword
#14
//myArrayLen is initialized to length of the array
myArrayLen initword
#4
//add this constant
const initword
#3
//jump on neg
CCNEG
initword
#1
!dataEnd
!textStart
0xA0
move #myArray
%r1
//%r1 points to element 0
//check array limit (use myArrayLen as count)
startLoop:
sub
#1
myArrayLen
//jump on negative
jumpct
CCNEG
endProg
//get next array element
//myArray[i] -> r2
move %r1(0)
%r2
//add 3 to each element of array.
//3 + r2 -> r2
add
const %r2
//modified element -> myArray[i]
move %r2
%r1(0)
//update ptr address
add
#1
%r1
jumpa startLoop
endProg:
halt
!textEnd
!stackStart
0xE0
!stackEnd
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Programming Fragments Examples
Notes:
SP is mapped into r3.
Here are some examples using different addressing modes. Literal cannot be destination.
op1 address mode register direct:
add
%r1
%r1 // r1 + r1 -> r1
add
%r1
varx // r1 + M[varx] -> M[varx]
op1 literal:
add
#20
%r1 //20 + r1 -> r1
add
#200 vary //200 + M[vary] -> M[vary]
move #myArray
%r2 //address of array -> r2
op2 mm-direct:
add
varx
vary // M[varx] + M[vary] -> M[vary]
add
200
vary // M[200] + M[vary] -> M[vary]
add
#varx vary // address of varx + M[vary] -> M[vary]
op2 register-indirect with offset:
add
%r1(0) %r2(10) //M[ r1 + 0] + M[r2 + 10] -> M[r2 + 10]
//jsr, jumpct, jumpcf, jumpa use “jump addressing mode”
//jump addressing mode: the address can be specified by a label
//or by a numeric address (with no # symbol).
jsr
200 //means push ret address, and 200-> PC
jsr
#200 //assembler error
jsr
startSub //means push return address, and startSub -> PC
jumpa op1 //op1 jump-mode
jumpct op1
op2 // op2 jump-mode
jumpcf op1
op2 // op2 jump-mode
//For jumpct, jumpcf, op1 can be any of the four basic addressing modes.
In/out
in
op1 //op1 can be reg-direct, mm-direct, reg-indirect with offset.
out
op1 // op1 can be any of the four basic addressing modes.
Push/pop
pop
op1 //op1 can be reg-direct, mm-direct, reg-indirect with offset.
push op1 //op1 can be any of the four basic addressing modes.
Some clarifications
move arr1
%r0 //M[arr1] -> %r0, mem mode
move 200
%r0 //M[200] -> %r0, mem mode
move #200
%r0 //200 -> %r0,
lit mode
move #arr1 %r0 //arr1 -> %r0,
lit mode
Control flow instructions:
jumpa 200
//200 -> PC,
jumpa loop //loop -> PC,
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jump mode
jump mode
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