Step 1 — Clock module
I started where many of us did, the clock module
Building Ben Eater's SAP-1 design
Step 2 — A & B registers
Then the registers, this is how I soldered resistors onto LEDs to save breadboard space
Step 3 — ALU
I guess my wiring was still pretty organized
Step 4 — Output
I added Decimal and Hexidecimal modes with an extra address line on the EEPROM
Step 5 — RAM
Yay memory
Step 6 — Control Logic + Flags
I also added LEDs to view all of the control lines
Upgrading the computer with my own designs!
Step 7 — Bigger Program Counter
Made a 16 bit program counter, not too much to talk about here
Step 8 — Bigger Memory
64k memory upgrade, 0x0000 through 0x7fff is ROM, 0x8000 to 0xffff is ram. There is also a setup to connect an arduino that can flash to the rom.
Step 9 — Better registers and ALU
Registers A through D plus an ALU supporting bitwise functions, shifting, adding, subtracting, etc with zero and carry flags for all those functions
Step 10 - Bigger control logic + output registers
Bigger control logic for all the control lines + 4 bit microcode counter. Also two 8-bit output registers, built the same as the A-D registers except they can't output back to the bus
Step 11 - Stack pointer and inputs
An 8 bit stack pointer (used in the range 0x8100 to 0x81ff in ram), and two 8 bit input registers.
Step 12 - interrupts
I technically got interrupts working, but theres a lot to improve with the setup, it was a very hacky way to do it. In the video example here the program just increments the A register, and the interrupt just increments the B register, a simple program I made to test the interrupts.
Step 13 - PS2
Got a ps2 keyboard interface working!
Step 14 - Back up and running
The computer had been in a box for ~2.5 years and I finally got it out and got it running again!
Next Steps
so, whats next?
First I need to get the arduino flashing to memory working again, and make the interrupts setup a little less jank, but after that??? Maybe a serial interface? Maybe I finally make some schematics for this thing?
Assembly
so, what does the instruction set for this thing look like? here is the instruction set as defined for customasm
NOP => 0x00
HLT => 0x01
LDA {value: u16} => 0x02 @ value`16
LDA {value: u16}, B => 0x03 @ value`16
LDA {value: u16}, C => 0x04 @ value`16
LDA {value: u16}, D => 0x05 @ value`16
LDA CD => 0x06
STA {value: u16} => 0x08 @ value`16
MVI A, {value} => 0x0A @ value`8
MVI B, {value} => 0x0B @ value`8
MVI C, {value} => 0x0C @ value`8
MVI D, {value} => 0x0D @ value`8
MOV A, B => 0x0E
MOV A, C => 0x0F
MOV A, D => 0x10
MOV B, A => 0x11
MOV B, C => 0x12
MOV B, D => 0x13
MOV C, A => 0x14
MOV C, B => 0x15
MOV C, D => 0x16
MOV D, A => 0x17
MOV D, B => 0x18
MOV D, C => 0x19
ADD B => 0x1A
ADD C => 0x1B
ADD D => 0x1C
ADI {value} => 0x1D @ value`8
SUB B => 0x1E
SUB C => 0x1F
SUB D => 0x20
SUI {value} => 0x21 @ value`8
ANA B => 0x22
ANA C => 0x23
ANA D => 0x24
ANI {value} => 0x25 @ value`8
ORA B => 0x26
ORA C => 0x27
ORA D => 0x28
ORI {value} => 0x29 @ value`8
XRA B => 0x2A
XRA C => 0x2B
XRA D => 0x2C
XRI {value} => 0x2D @ value`8
INR A => 0x2E
INR B => 0x2F
INR C => 0x30
INR D => 0x31
DCR A => 0x32
DCR B => 0x33
DCR C => 0x34
DCR D => 0x35
ROR A => 0x36
ROR B => 0x37
ROR C => 0x38
ROR D => 0x39
ROL A => 0x3A
ROL B => 0x3B
ROL C => 0x3C
ROL D => 0x3D
RRC A => 0x3E
RRC B => 0x3F
RRC C => 0x40
RRC D => 0x41
RLC A => 0x42
RLC B => 0x43
RLC C => 0x44
RLC D => 0x45
JMP {value} => 0x46 @ value`16
JZ {value} => 0x47 @ value`16
JNZ {value} => 0x48 @ value`16
JC {value} => 0x49 @ value`16
JNC {value} => 0x4A @ value`16
OUT 1 => 0x4B
OUT 2 => 0x4C
JSR {value} => 0x4D @ value`16
RTS => 0x4E
IN 1 => 0x4F
IN 2 => 0x50
PHA => 0x51
PHB => 0x52
PHC => 0x53
PHD => 0x54
PLA => 0x55
PLB => 0x56
PLC => 0x57
PLD => 0x58
STA {value: u16}, B => 0x59 @ value[7:0] @ value[15:8]
STA {value: u16}, C => 0x60 @ value[7:0] @ value[15:8]
STA {value: u16}, D => 0x61 @ value[7:0] @ value[15:8]
PHF => 0x62
PLF => 0x63