85 8 bit CPU

85 : 8 bit CPU

Image path is broken

How it works

It has an 8-bit accumulator, a 12-bit PC, 2 13-bit index registers and a carry bit. The main limitations are the 6/8-bit external bus - it’s designed to run with an external SRAM and a 12-bit address latch, code is loaded externally. There are 33 instructions. each 1, 2 or 3 bytes: 0v: add a, v(x/y) - sets C 1v: sub a, v(x/y) - sets C 2v: or a, v(x/y) 3v: and a, v(x/y) 4v: xor a, v(x/y) 5v: mov a, v(x/y) 6v: movd a, v(x/y) 70: add a, c 71: inc a 72: swap x, y 73: ret 74: add y, a 75: add x, a 76: add y, #1 77: add x, #1 78: mov a, y 79: mov a, x 7a: mov b, a 7b: swap b, a 7c: mov y, a 7d: mov x, a 7e: clr a 7f: mov a, p 8v: nop 9v: nop av: movd v(x/y), a bv: mov v(x/y), a cv: nop dv: nop ev: nop f0 HL: mov a, #HL f1 HL: add a, #HL f2 HL: mov y, #EEHL f3 HL: mov x, #EEHL f4 HL: jne a/c, EEHL if EE[4] the test c otherwise test a f5 HL: jeq a/c, EEHL if EE[4] the test c otherwise test a f6 HL: jmp/call EEHL f7 HL: nop Memory is 4096 8-bit bytes, references are a 3 bit (8 byte) offset from the X or Y index registers - the general idea is that the Y register points to a register scratch pad block (a bit like an 8051) but can also be repurposed for copies when required. There is an on-chip SRAM block for data access only (addressed with the MSB of the data address) - mostly just to soak up any additional gates. There is also a 3-deep hardware call stack. Assembler is here: https://github.com/MoonbaseOtago/tt-asm

How to test

needs a 7-bit external address latch and SRAM


# Input Output
0 clock data_out_0
1 reset data_out_1
2 ram_data0 data_out_2
3 ram_data1 data_out_3
4 ram_data2 write_data_n
5 ram_data3 write_ram_n
6 io_data0 a
7 io_data1 strobe