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Computer Architecture

Unicorn CPU emulator

Unicorn CPU emulator

Motorola 68000

Note: This information is generally geared to the 68000 specifically (for use with the Sega Genesis), later processor models have more/enhanced instructions :-)


field desc
#X immediate (raw) data
ea effective address (data register or memory address)
An address register
Dn data register
Rn any register
Mnemonic Sizes Desc
add ea,ea b,w,l Add source to destination (either needs to be Dn)
addi #X,ea b,w,l Add source to destination (either needs to be Dn)
bCC label b,w See Condition Code test table (68020+ supports long)
bchg Dn,ea b,l test a bit in destination operand, set Z code, flip bit
bclr Dn,ea b,l test a bit in destination operand, set Z code, set bit to 0
bset Dn,ea b,l test a bit in destination operand, set Z code, set bit to 1
clr ea b,w,l Zero out destination
eor Dn,ea b,w,l Exclusive OR (XOR)
exg Rn l Exchange registers
lea ea,An l Load effective address into an address register
move ea,ea b,w,l Move source ea to dest ea (assembler converts to movea if uses direct register)
movea ea,An w,l Move ea to address register
moveq #x, Dn l Move immediate long to data register
not ea b,w,l Flip bits of effective address
swap Dn w Swap upper word with lower word
tst ea b,w,l test an operand (sets Z if operand zero)

Only byte-sizes can use odd addressing (word+long crash).

Condition Code Tests

Mnemonic Condition
CC(HI) Carry Clear
CS(LO) Carry Set
EQ Equal
GE Greater or Equal
GT Greater Than
HI High
LE Less or Equal
LS Low or Same
LT Less Than
MI Minus
NE Not Equal
PL Plus
VC Overflow Clear
VS Overflow Set

Condition codes

code condition
X ?
N Negative
Z Zero
V Overflow
C Carry

Motorola assembly syntax

; mnemonic.size source,destination

Size is ‘b’ (byte), ‘w’ (word), or ‘l’ (long)

operand desc
$00000042 memory address of 00000042
#42 decimal 42 literal
#$42 hexadecimal 42 literal
#%01000010 binary 42 literal
a0 address register 0
d0 data register 0
$42(a0) add offset of 42 to address register 0
(a0)+ increment address register 0 (register incremented after instruction)
-(a0) decrement address register 0 (register decreased before instruction)


Tessellated Intelligence System

System details

Massively parallel computer architecture comprised of non-uniformly interconnected heterogeneous nodes

Node Type T21 - Basic Execution Node

TIS-100 Registers

Register Notes
ACC Accumulator register.
BAK Non-addressible temporary storage for values in ACC.

TIS-100 Ports

Port Notes
LEFT, RIGHT, UP, DOWN Communication register for topologically adjacent nodes. Blocks indefinitely if used until transaction with other node occurs.
ANY Reads or writes a value that becomes available on ANY port. (TODO - deterministic which node reads ANY value first?)
LAST Refers to the port last read or written using the ANY pseudo-port.

TIS-100 Instruction Set

Instruction Description
NOP pseudo-instruction converted to ADD NIL
MOV SRC, DST SRC is read and written to DST
SWP Exchange ACC and BAK
SAV Write ACC to BAK
ADD SRC Add SRC to ACC, store result in ACC
SUB SRC Subtract SRC from ACC, store result in ACC
NEG The value of ACC is arithmetically negated
JMP LABEL Transfer execution to instruction after LABEL
JEZ LABEL Transfer execution to instruction after LABEL if value of ACC is zero.
JNZ LABEL Transfer execution to instruction after LABEL if value of ACC is not zero.
JGZ LABEL Transfer execution to instruction after LABEL if value of ACC is greater than zero.
JLZ LABEL Transfer execution to instruction after LABEL if value of ACC is less than zero.
JRO SRC Transfer execution to instruction at offset specified by SRC relative to current instruction.
HCF Halt and Catch Fire (undocumented)

Node Type T30 - Stack Memory Node

Node Type T31 - Random Access Memory Node


Visualization Module

Intel x86


ins desc
push OP Push operand to stack
pop OP Pop operand off stack
call ADDR Call function by setting EIP to ADDR. Next instruction following call pushed to stack
ret Pop return address from stack, set EIP to address
int OP Generate software interrupt


Starts in 16-bit Real Mode, for compatibility reasons

Zero the data segment registers first thing, since their content is unknown.

The BIOS transfers the first 512 bytes of data from the device into 0x7c00. The last two bytes need to be 0x55 and then 0xAA to be considered a valid bootsector.

Real Mode




Cyrix coma bug



Sega Genesis/Mega Drive

The Genesis (Mega Drive in Japan) has 2 on-board processors, a 68000 @ 8MHz and a z80 @ 4MHz (to handle sound processing).

68000 Memory map

Start address End address Description 32X SH-2 address
0x000000 0x3FFFFF Cartridge ROM/RAM 0x2000000-0x23FFFFF
0x400000 0x7FFFFF Reserved (used by the Mega-CD and 32X)  
0x800000 0x9FFFFF Reserved (used by the 32X)  
0x840000 0x85FFFF 32X frame buffer 0x4000000-0x401FFFF
0x860000 0x87FFFF 32X frame buffer overwrite mode 0x4020000-0x403FFFF
0x880000 0x8FFFFF 32X cartridge ROM (first 512kB bank only)  
0x900000 0x9FFFFF 32X cartridge bankswitched ROM (any 512kB bank, controlled by 32X registers)  
0xA00000 0xA0FFFF Z80 memory space  
0xA10000 0xA10001 Version register  
0xA10002 0xA10003 Controller 1 data  
0xA10004 0xA10005 Controller 2 data  
0xA10006 0xA10007 Expansion port data  
0xA10008 0xA10009 Controller 1 control  
0xA1000A 0xA1000B Controller 2 control  
0xA1000C 0xA1000D Expansion port control  
0xA1000E 0xA1000F Controller 1 serial transmit  
0xA10010 0xA10011 Controller 1 serial receive  
0xA10012 0xA10013 Controller 1 serial control  
0xA10014 0xA10015 Controller 2 serial transmit  
0xA10016 0xA10017 Controller 2 serial receive  
0xA10018 0xA10019 Controller 2 serial control  
0xA1001A 0xA1001B Expansion port serial transmit  
0xA1001C 0xA1001D Expansion port serial receive  
0xA1001E 0xA1001F Expansion port serial control  
0xA10020 0xA10FFF Reserved  
0xA11000   Memory mode register  
0xA11002 0xA110FF Reserved  
0xA11100 0xA11101 Z80 bus request  
0xA11102 0xA111FF Reserved  
0xA11200 0xA11201 Z80 reset  
0xA11202 0xA12FFF Reserved  
0xA13000 0xA130FF TIME registers; used to send signals to the cartridge  
0xA130F1   SRAM access register  
0xA130F3   Bank register for address 0x80000-0xFFFFF  
0xA130F5   Bank register for address 0x100000-0x17FFFF  
0xA130F7   Bank register for address 0x180000-0x1FFFFF  
0xA130F9   Bank register for address 0x200000-0x27FFFF  
0xA130FB   Bank register for address 0x280000-0x2FFFFF  
0xA130FD   Bank register for address 0x300000-0x37FFFF  
0xA130FF   Bank register for address 0x380000-0x3FFFFF  
0xA14000 0xA14003 TMSS “SEGA”  
0xA14104 0xBFFFFF Reserved  
0xC00000 0xC00001 VDP data port  
0xC00002 0xC00003 VDP data port (mirror)  
0xC00004 0xC00005 VDP control port  
0xC00006 0xC00007 VDP control port (mirror)  
0xC00008 0xC00009 VDP H/V counter  
0xC0000A 0xC0000F VDP H/V counter (mirror)  
0xC00011   PSG output  
0xC00013 0xC00017 PSG output (mirror)  
0xC0001C 0xC0001D Debug register  
0xC0001E 0xC0001F Debug register (mirror)  
0xC00020 0xFEFFFF Reserved  
0xFF0000 0xFFFFFF 68000 RAM  

Z80 memory map

Start End Description
0000h 1FFFh Z80 RAM
2000h 3FFFh Reserved
4000h   M2612 A0
4001h   M2612 D0
4002h   M2612 A1
4003h   M2612 D1
4004h 5FFFh Reserved
6000h   Bank register
6001h 7F10h Reserved
7F11h   PSG
7F12h 7FFFh Reserved
8000h FFFFh 68000 memory bank

SegaCD / MegaCD

Start address End address Description
0x000000 0x01FFFF MegaCD BIOS ROM
0x020000 0x03FFFF MegaCD “Program RAM” Bank Access
0x200000 0x23FFFF MegaCD “WORD RAM”
0xA12000 0xA120XX MegaCD “Gate Array”
0xFFFD00 0xFFFDFF MegaCD Interrupt/Exception vectors


Flynn’s Taxonomy

acronym desc examples
SISD Single instruction, single data Old uniprocessors
MISD Multiple instruction, single data Fault-tolerant architectures, somewhat uncommon
SIMD Single instruction, multiple data array processors, GPUs
MIMD Multiple instruction, multiple data Multi-core superscalar, distributed systems

Dennard Scaling

Voltage and power scale down in length. If transistor density doubles, power consumption remains the same. However, at smaller sizes (started to see around 2006) current leakage is harder to deal with, which creates thermal runaway.