OPC1 is a minimal implementation of a One Page Computer which fits into a single Xilinx XC9572 CPLD.
The OPC1 is an accumulator based computer with an 8 bit datapath and a 11 bit address space. The computer supports 3 addressing modes:
The OPC has just one flag register: the carry (C) flag. Branching instructions dependent on the state of this flag and the state of the accumulator are available.
All instructions are encoded in a fixed two byte format consisting of an opcode and an operand ::
bbbbb ooo : oooooooo \ \ \_______ 8 bit operand for Immediate/Implied Instructions, or \ \________\______ 11 bit operand for Direct/Indirect Instructions \______________________ 5 bit opcode
On reset the CPU starts execution at address 0x100 to leave page zero free for variables which can be accessed by the 8 bit pointers.
|Mnemonic||Opcode||Function (Imm. Mode)||Opcode||Function (Direct or Ind.Mode)||Carry|
|AND[.i]||1.0000||A <- A & n||0.0000||A <- A & (n)||0|
|LDA[.i]||1.0001||A <- n||0.0001||A <- (n)||-|
|NOT[.i]||1.0010||A <- ~n||0.0010||A <- ~(n)||-|
|ADD[.i]||1.0011||A <- A+n+C||0.0011||A <- A+(n)+C||arith|
|LDA.p||-||-||0.1001||A <- ((n))||-|
|STA[.p]||1.1000||(n) <- A||0.1000||((n)) <- A||-|
|JPC||1.1001||PC <-n if C else PC+2||-||-||-|
|JPZ||1.1010||PC <-n if (ACC==0) else PC+2||-||-||-|
|JP||1.1011||PC <- n||-||-||-|
|LXA||1.1110||Swap Acc with Link||-||-||X|
Subroutine call and returns are handled by the LXA, JSR and RTS instructions. When a JSR instruction is issued the upper 3 bits of the PC are placed into a link register and the lower 8 bits are stored in the accumulator. On entering the subroutine the accumulator value should be saved and then using the LXA instruction the link register bits can be moved to the accumulator and saved also. On leaving a subroutine the return address needs to be retrieved by the reverse of this process. First get the upper return address bits into the accumulator, then transfer them to the link register using LXA and finally get the lower return address bits into the accumulator. Executing the RTS will now set the PC to the 11 bit value made by concatenating link register with accumulator.
NB In order to fit in the 9572, the bottom bit of the link register doubles up as the carry flag, so any instruction altering the link will invalidate the carry and vice versa.