Modular Computing CPU design v1

gistfile1.txt

AREIA-1
------------

16 registers, 16 bits each,
        0 = always 0
        15 = SP

Memory is treated as little-endian.
 
OP1 in {move, cmp, add, sub, xor, or, and}
        OP1.# @x, @y
        OP1.# @x, #i
 
OP2 in {asl, asr, lsr, rol, ror, rcl, rcr}
        OP2.# @x, @y
        OP2.# @x, #i
 
OP3 in {jz, jnz, jc, jnc, jv, jnv, js, jns, jmp, jsr}
        OP3 $baaaa
        OP3 $aa000, @x 

OP4 in {ld, st}
        OP4.# @x, $Faaaa
        OP4.# @x, $aaa00, @y
        OP4.# @x, $baaaa, @y

SPECIAL OPCODES (xxxx0000):
00000000: NOP
00100000: RET
01000000: POPF
01100000: PUSHF
10000000: CLI
10100000: SEI
11000000: HLT

ooo0xxxx iiiiiiii: OP1.b @x, #i
ooo1xxxx iiiiiiii iiiiiiii: OP1.w @x, #i
11100ooo yyyyxxxx: OP1.b @x, @y
11110ooo yyyyxxxx: OP1.w @x, @y
11101ooo yyyyxxxx: OP2.w @x, #y
11111ooo yyyyxxxx: OP2.w @x, @y
 
11100111 oooobbbb aaaaaaaa aaaaaaaa: OP3 $baaaa
11110111 ooooxxxx aaaaaaaa: OP3 $aa000, @x
11101111 001oxxxx aaaaaaaa aaaaaaaa: OP4.b @x, $Faaaa
11111111 001oxxxx aaaaaaaa aaaaaaaa: OP4.w @x, $Faaaa
11101111 010oxxxx yyyybbbb aaaaaaaa: OP4.b @x, $baa00, @y
11111111 010oxxxx yyyybbbb aaaaaaaa: OP4.w @x, $baa00, @y
11101111 011oxxxx yyyybbbb aaaaaaaa aaaaaaaa: OP4.b @x, $baaaa, @y
11111111 011oxxxx yyyybbbb aaaaaaaa aaaaaaaa: OP4.w @x, $baaaa, @y

FLAGS:
    bit     description
---------------------------------------
    0       ZERO (CMP equality)
    1       CARRY (whether carrying a bit, for subtract follow Z80 impl.)
    2       OVERFLOW (whether operation has overflowed)
    3       SIGNED (whether >= 0x8000 or >= 0x80 for b)
    4       INTERRUPT (set to 1 to enable)
    5-15    Reserved.

MEMORY MAP:

    start       end         description
---------------------------------------------------------
    $00000      $EFFFF      Main memory area (RAM)
    $F0000      $FBFFF      Mirror of $00000 - $0BFFF
    $FC000      $FCFFF      Reserved for future updates
    $FD000      $FDFFF      I/O area (described below)
    $FE000      $FFF7F      ROM
    $FFF80      $FFFFF      CPU registers (described below)

I/O AREA (16 x 256 bytes): $FDxyy (x - device ID (0-F), y - as below:)

    start       end         type        description
---------------------------------------------------------
    $00         $01         short       Vendor ID ($0000-$7fff assigned by me, $8000-$feff usable by anyone, $ffff indicates nothing is connected)
    $02         $02         byte        Device ID (dependent on vendor ID)
    $03         $03         byte        Device class ($00-$7f assigned by me, $80-$fe usable by anyone, $ff - check via vendor/device ID)
    $04         $FF         ...         Dependent on device class, device registers/data

Device indexes 0 to 14 are devices connected to the bus. Device index 15 is the mainboard:

    start       end         type        description
---------------------------------------------------------
    $04         $07         byte[4]     A 32-bit bit array defining what functionality is supported by this mainboard.
                                        Bit 0 - MMU     Bit 1 - DMA
    $08         $08         byte        [MMU] Top 8 bits of the bank.
    $09         $09         byte        [MMU] Write to set the bank to real memory area X, read to get the current area of the bank.

MMU MEMORY AREAS:
0-239 - real memory ($00000-$EFFFF)
240-241 - ROM
242 - I/O
243-255 - RESERVED

$FFF80-$FFFFF is hardwired and cannot be moved with the MMU.

CPU REGISTERS: $FFF80-$FFFFF

    start       type        description
---------------------------------------------------------
    $FFF80      byte[3]     Bits 0-19 point to the address of the interrupt vector. Bits 20-23 are reserved.
    $FFF83      byte        Top 8 bits of stack pointer (Stack starts at $xxFFF and makes its way up up to $0FFFF more)
    $FFF84      byte[4]     32 bits, one for each interrupt line. Interrupt lines are configured from within devices. To clear an interrupt line, write the bit as set to the proper address. Interrupt lines 0-3 are reserved by the CPU.

(The CPU will also include the timer in a future revision)

Proposal.
$FFFFC.w = reset address (read from ROM initially, then stored in registers so you can change it)
$FFFFE.b = reset address, upper byte