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CPU made with VHDL

Instructions

func: 6 bits, rs: 4 bits, rt: 4 bits, adr: 8 bits, ra: 4 bits

Instruction Format Operation
RT $func, $rs, $rt 0b00[func][rs][rt] $rs = $rs $func $rt
LD $rs, $adr 0b0100[rs][adr___] $rs = memory[$adr]
LDR $rs, $rt, $ra 0b0101[rs][rt][ra] $rs = memory[$rt + $ra]
ST $rs, $adr 0b0110[rs][adr___] memory[$adr] = $rs
STR $rs, $rt, $ra 0b0111[rs][rt][ra] memory[$rt + $ra] = $rs
JZ $rs, $adr 0b1001[rs][adr___] pc = $rs == 0 ? $adr : pc
JNZ $rs, $adr 0b1010[rs][adr___] pc = $rs != 0 ? $adr : pc
JU $adr 0b10000000[adr___] pc = $adr
SB $adr 0b01100000[adr___] bsr = $addr
Func Operation
000010 $rs = $rs + $rt
010011 $rs = $rs - $rt
000111 $rs = $rt - $rs
011111 $rs = $rs + 1
001110 $rs = $rs - 1
000000 $rs = $rs and $rt
010101 $rs = $rs or $rt
001101 $rs = not $rs
110001 $rs = not $rt
001100 $rs = $rs
110000 $rs = $rt

Registers

Name Description
PC Program counter
MAR Memory address register
MBR Memory buffer register
AC[16] Accumulator
IR Instruction register
BSR Bank select register

Accumulators

Name Number Description
$0 0 Constant 0
$at 1 Assembler temp
$v0 2 Return value
$a0~$a1 3~4 Arguments
$t0~$t7 5~12 Temporaries
$sp 13 Stack pointer
$fp 14 Frame pointer
$ra 15 Return address

State transition

State \ Instruction RT LD LDR ST JZ JU SB
e0 Load_MAR, Gate_PC Load_MAR, Gate_PC Load_MAR, Gate_PC Load_MAR, Gate_PC
e1 Load_Memory Load_Memory Load_Memory Load_Memory
e2 Load_AC, Gate_AC, ALU = $func Load_MAR, Gate_MBR Load_AC, Gate_AC, ALU = PASS, adr1 = at, adr2 = rt Load_BSR, Gate_MBR
e3 Inc_PC Load_Memory Load_AC, Gate_AC, ALU = ADD, adr1 = at, adr2 = ra Inc_PC
e4 Load_AC, Gate_MBR, ALU = PASS Load_MAR, Gate_A, adr1 = at
e5 Inc_PC Load_Memory
e6 Load_AC, Gate_MBR, ALU = PASS, adr1 = rs
e7 Inc_PC

OS: Calculatos

Subroutines

write

Write a string to the screen.

Arg Description
adr Head address of the string.s
len Length of the string. 0 < len <= 6 
    LD $a0, adr
    LD $a1, len
    LD $ra, x
    JU write
x:  //

// write
write: LD $t0, chars
       LD $t2, display
       AND $t3, $0 // offset
y:     LDR $t1, $a0, $t0 // load char
       STR $t1, $t2, $t3 // display char
       INCR $t3, $0
       DECR $a1, $0
       JNZ $a1, y
       JU $ra // return

// 7 seg decoder char map
chars: 01111110 // 0
       00110000 // 1
       01101101 // 2

display: // memory mapped i/o

read

Read a number from the keyboard.

Arg Description
adr Address of the buffer
len Buffer size 
    LD $a0, adr
    LD $a1, len
    LD $ra, x
    JU read
x:  //

read: LD $t0, chars
      LD $t1, display
      AND $t2, $0 // offset

Flow

Key(3): Input number Key(2): Input operator Key(1): Calculate

  1. Display "Calculator" on the screen for 2 seconds
  2. Wait for input
  3. If key(3) is pressed, display the number on the screen