MINISC - Minimal Instruction Set Computing

.3v   data
  | ||||||||
  ##########
  # minisc #
  ##########
  | ||||||||
gnd   addr

The MINISC architecture is an emulator written in C, of a fictional minimal Von Neumann Turing Machine. A MINISC CPU have:

• 4 internal registers.
• 8 data pins.
• 8 address pins.
• one pin for .3 Volts input.
• ground pin.

It can address up to 256 bytes, with no bank switching. The MINISC CPU don't have I/O ports, but it can interface with external devices via shared RAM.

Instruction Layout

o    s t
0000.0000

o = opcode
t = target
s = source

The following table shows the encoding of the high nibble for each instruction.

add sub mul div  |  0 1 2 3
mod inc dec lur  |  4 5 6 7
and  or xor cmp  |  8 9 A B
biz bis biq big  |  C D E F

Registers

op rg pc st

op = instruction
rg = accumulator, register
pc = program counter
st = status

The MINISC architecture gives you full access to the status and program counter registers, both as source and target. You can manually clear the status registers, or jump by loading the address on the program counter.

When the source and/or target of the current instruction is OP, it refers to the next instruction. That's how the CPU writes to RAM.

The following table shows how the instruction is encoded for the target and source lower nibble.

t\s  op rg pc st
op | 0  1  2  3
rg | 4  5  6  7
pc | 8  9  A  B
st | C  D  E  F

Status bits

zer  sml  eql  gtr
neg  car  skp? int?

The skp flag tells the MINISC cpu that the next byte should not be interpreted as a instruction, but as data. The 'int' status flag sets the OP register to 0xf0, making the CPU jump to the last "page". But I'm not sure if they're really necessary yet. So let they be placeholders.

Here's a list of all the operators and which bits they may set on the status register. The values marked with a question mark need to be checked.

add -> zero  | equal | carry
sub -> zero  | equal | negative
mul -> zero  | equal | carry
div -> equal
mod -> zero
inc -> zero (overflow or carry)
dec -> zero (underflow or negative)
lur -> none
and -> zero | equal | greater? | smaller?
or  -> zero | equal | greater? | smaller?
xor -> zero | equal? | greater? | smaller?
cmp -> smaller | equal | greater
biz -> none
bis -> none
biq -> none
big -> none

Macros

The MINISC CPU don't have a jmp instruction, But who needs it? you can load the desired address directly into the PC register.

jmp $AD = ldr PC
          $AD