Twidl.txt

Twidl - A 6502-inspired 8-bit microprocessor

Each instruction listed has an 8-bit value. A letter denotes a value used by the instruction.

0000 CCCC - INT - Sends an interrupt of the given 4-bit code if bit 6 of S is set. In a compliant implementation, code 0 must be a noop, and code 15 must jump PROG to the address specified at FFFE and FFFF.
0001 0RRR - TES - Sets bit 0 of S to whether the register is zero, sets bit 7 of S to bit 7 of the register.
0001 1III - CHK - Sets bit 3 of S to the bit of A at the index specified by the 3-bit number in I.
0010 0000 - NAN - Takes the NAND of the U register and the A register, and stores it in the A register.
0010 0001 - IOR - ^ but IOR
0010 0010 - AND - ^ but AND
0010 0011 - NOR - ^ but NOR
0010 0100 - ADD - Adds the value in the U register and the A register, and stores it in the A register. Sets bit 1 if unsigned overflow occurred. 
0010 0101 - SUB - ^ but arithmetically negates A first
0010 0110 - NOT - Logically negates the value in the A register and stores it back in that register.
0010 0111 - NEG - Arithmetically negates the value in the A register and stores it back in that register.
0010 1000 - SHL - Shifts the value in the A register left by one bit. Arithmetic shift if bit 5 of S is set, logical otherwise.
0010 1001 - SHR - Shifts the value in the A register right by one bit. Arithmetic shift if bit 5 of S is set, logical otherwise.
0010 1010 - ROL - Rotates the value in the A register left by one bit.
0010 1011 - ROR - Rotates the value in the A register right by one bit.
0010 1100 - INC - Increments the value in the A register. Sets bit 1 if unsigned overflow occurred.
0010 1101 - DEC - ^ but decrements instead
0010 1110 - LOW - Clears the top 4 bits of A.
0010 1111 - HIG - Sets the bottom 4 bits of A to the top 4 bits, then clears the top.
0011 0RRR - PSH - Pushes the value in the given register to the stack.
0011 1RRR - POP - Pops the value in the given register from the stack.
0100 BIII - SET - Uses 3 bits into an index into S, and sets that bit to the final bit.
0101 0RRR - CPY - Stores the value of the given register to the C register.
0101 1RRR - PST - Loads the value of the C register to the given register.
0110 BIII - BRA - Uses I as a index into S, and if B matches that bit, swaps the current program index with the address stored in L and H.
0111 AAAA - RJP - Swaps the low and high bytes of the current program index with the address stored in L and H, plus a signed 8-bit value with the high nibble set to the low nibble of the instruction, and the low nibble set to all 0's.
1VVV VVVV - IMM - Stores a 8-bit value into the C register, with the highest bit determined by bit 4 of S.

Registers:
U - General
A - Used by ALU
X - Stack pointer (the hardware stack is stored as 256 bytes separate from the RAM, and grows downward)
C - Used by CPY and PST
L - Address low byte 
H - Address high byte 
I - Mapped I/O to RAM using L and H as an address
S - 8 bit bitfield - NIASQUCZ - Zero, Carry, User, Query, Immediate Sign, Arithmetic Shift, Interrupt Enabled, Negative
PROG - Inaccessible by normal means, used with BRA and JMP
All registers start as 0, except PROG, which starts at 0x0200, and S, which starts as 0b01000000.