# TR-8

**Important**: this is a work in progress with pre-alpha quality. Not all the information in this document is final, complete or even accurate.

TR8 is an 8-bit fantasy console inspired by MIPS, Z80 and 6502 real world CPUs.

## Specs

```
Display:  128 x 128 pixels 16 colors
Memory:   64K
CPU:      TR-8, 4M VM instr/sec
Sound:    TBD
```

Other features:

* programmable in ASM
* 16-bit registers: stack pointer (SP) and program counter (PC)
* 8-bit registers: 4 general purpose registers (a, b, x, y), flags register (F)
* frame interrupt (60Hz) and external IO interrupt (planned)
* port based IO
* hardware blitter

Memory map:

```
0x0000
        program (48896 bytes)
0xbf00
        frame buffer (16K)
0xff00
        frame interrupt vector
0xff02
        IO interrupt vector
0xff04
        stack (251 bytes)
0xffff
```

## How to build

It requires SDL2 and SDL_Mixer.

On Linux install a C compiler and the development packages for the requirements.

Example on Debian/Ubuntu using `sudo`:

    sudo apt install build-essential libsdl2-dev libsdl2-mixer-dev

Then run `make`.

This will result in two binaries:

 - **tr8as**: the TR8 assembler
 - **tr8vm**: the TR8 VM Player that can run `tr8` files

You can then compile and run the example with `make example`.

## Detailed specs

### Ports

| Port | Type | Result |
| --- | --- | --- |
| 0xb0 | write | Blitter control |
| 0xb1 | write | Blitter settings |
| 0xf0 | read | Status of the controller 1 |
| 0xf1 | read | Status of the controller 2 |

### The blitter

TR8 implements a hardware blitter that can be controlled by writing to ports.

On a successful port call, the read value equals to the port number, so the call preserves the port register.

To draw with the blitter the following steps are required:

1. put the blitter in settings mode using control port `0xb0`
2. provide an address, and screen coordinates (x, y, width and height) using settings port `0xb1`
3. set the blitter in one of the draw modes using the control port `0xb0`

The address will be the source data in write mode, and the destination in read mode.

Step 3 can be repeated as many times as needed, the blitter will keep the settings until the control port is set in settings mode.

The settings byte is as follow:

| Bits | Effect
| --- | --- |
| `x0000000` | Set blitter in settings mode |
| `0000000x` | Write to video RAM |
| `000000x0` | The source includes a transparent bit |
| `00000x00` | Read from video RAM |
| `0xxxx000` | Unused |

Example:

```asm
    ; blitter in settings mode
    ld x, 128
    ld a, 0xb0
    port a, x

    ; setup
    inc a
    ; source
    ld x, <sprite
    port a, x
    ld x, >sprite
    port a, x
    ; destination (56, 56)
    ld x, 56
    port a, x
    port a, x
    ; size 16x16
    ld x, 16
    port a, x
    port a, x

    ; now blit
    dec a
    ld x, 3
    ; 3: write with transparent color support
    port a, x
```

When bit 2 in the settings byte is set to 1 and the most significant bit in the
source data is set (e.g. `128`), that pixel won't be drawn (transparent).

### Controller

The controller support d-pad with 4 directions, 2 action buttons, select and start.

Read the port `0xf0` for the 8 1-bit flags reporting the state of the
controller 1, and `0xf1` for controller 2.

| Bits | Meaning |
| --- | --- |
| `0000000x` | a (fire 1) |
| `000000x0` | b (fire 2) |
| `00000x00` | d-pad up |
| `0000x000` | d-pad down |
| `000x0000` | d-pad left |
| `00x00000` | d-pad right |
| `0x000000` | select |
| `x0000000` | start |

### Palette

Current palette (not final) is the default EGA 16 color palette:

| Index | RGB (hex) |
| --- | --- |
| 0  | `0x000000` |
| 1  | `0x0000aa` |
| 2  | `0x00aa00` |
| 3  | `0x00aaaa` |
| 4  | `0xaa0000` |
| 5  | `0xaa00aa` |
| 6  | `0xaa5500` |
| 7  | `0xaaaaaa` |
| 8  | `0x555555` |
| 9  | `0x5555ff` |
| 10 | `0x55ff55` |
| 11 | `0x55ffff` |
| 12 | `0xff5555` |
| 13 | `0xff55ff` |
| 14 | `0xffff55` |
| 15 | `0xffffff` |

### Frame interrupt

When interrupts are enabled (IF not set), 60 times per second there will be a
frame interrupt:

* the PC is saved on the stack
* the F register (flags) is saved on the stack
* all the flags are cleared and IF is set
* the execution goes to the address in `0xff00` (the frame interrupt vector)
* the interruption handler code must return with `IRET`, that will restore
  the PC and the F register

By default the vector points to `0x0000` so it has to be setup before enabling
interrupts with `CIF`.

Example:

```asm
    ; setup an int handler so we
    ; can use the frame int o sync
    ld a, 0xff
    ld x, 0
    ; lower byte of the address
    ld b, <int_handler
    ld [a : x], b
    inc x
    # higher byte of the address
    ld b, >int_handler
    ld [a : x], b

    ; enable interrupts
    cif

loop:
    halt
    jmp loop

int_handler:
    ; just return
    iret
```

Because the flags are preserved, any code run in the interrupt handler only
needs to preserve the 4 registers and the stack.

### Labels and IDs

In `tr8as` an ID can have letters, numbers and special characters: `$_.#<>`,
but can't start with `>` or `<`, or match one of the reserved keywords.

A label is defined as an ID that ends in `:`.

### Instructions

All the instructions are 16-bit, with the exception of `JMP addr` and `CALL
addr` that use an extra 16-bit parameter for "addr". All the instructions take
the same time to run, and being 4 MIPS it can fit `66666` instructions in a
frame (at 60Hz).

There are no 16-bit registers, but they can be implemented with 2 registers,
and that is supported in some addressing modes with `[r1:r2]`.

For example:

```asm
    ; fill with color 15
    ld a, 15
    push a
    call fill
    pop a

    ; halt the CPU
    sif
    halt

    ;
    ; void fill(uint8_t color)
    ;
    ; fill frame-buffer with a color
    ;
fill:
    ; grab the color from the stack
    ld b, [sp + 2]
    ; 64 * 256 is 16K
    ld y, 64
    ; a:x is our 16-bit register
    ld a, 0xbf
    ld x, 0
fill_loop:
    ld [a : x], b
    inc x
    bno
    jmp fill_loop
    ; x overflows, so we increment a
    ; and decrement y (one 256 block done)
    inc a
    dec y
    bnz
    jmp fill_loop
    ret
```

Arithmetic operations on the "16-bit" register can be performed easily using
branching instructions:

```asm
    ; inc a:x
    inc x
    ; if x overflows we need to increase a
    bo
    inc a
```

#### Assembler directives

**.org addr**  
Set the address to that value. By default the starting address is `0x0000`.

**.db imm [, imm]**  
Literal byte. Label can be used with `<` prefix for the low byte of the address
and `>`for the high byte.

**.dw imm [, imm]**  
Literal word (16-bit value).

**.ds count, imm**  
Generates `count` bytes with value `imm`.

**.equ id value**  
Define an ID assigning a constant value. When the id is used, it will be replaced with its value.

**.include "filename"**  
Assemble the file at current position.

**.incbin "filename"**  
Read the file and add the content to the output at current position.

**.incpng "filename"**  
Read the PNG image and add the content to the output at current position. The
image colors will be matched with the TR8 palette and any non-matching color
will be condiered transparent and the index `128` will be used.

The blitter won't draw any pixel with index larger than 15.

#### Load and Store

**LD r1, r2**  
Load the value of r2 in r1.

**LD r1, imm**  
Load the immediate value in r1.

**LD [r1:r2], r3**  
Load the value of r3 in the memory address provided by r1:r2.

**LD r3, [r1:r2]**  
Load into r3 the value in the memory address provided by r1:r2.

**LD [SP + imm], r1**  
Load r1 into the stack on top address plus the immediate value.

**LD r1, [SP + imm]**  
Load into r1 the stack value on top address plus the immediate value.

#### Stack

**PUSH r1**  
Save the value of r1 on the top of the stack.

**POP r1**  
Restore the value of r1 by popping it from the top of the stack.

**PUSH F**  
Save the flags in the stack.

**POP F**  
Restore the flags from the top of the stack.

**XSP r1**  
Exchange the high byte of the stack pointer by the value in r1.

#### Logical

**AND r1, r2**  
Logical AND between r1 and r2, storing the result in r1.  
Affected flags: ZF, SF

**AND r1, imm**  
Logical AND between r1 and immediate value, storing the result in r1.  
Affected flags: ZF, SF

**OR r1, r2**  
Logical OR between r1 and r2, storing the result in r1.  
Affected flags: ZF, SF

**OR r1, imm**  
Logical OR between r1 and immediate value, storing the result in r1.  
Affected flags: ZF, SF

**XOR r1, r2**  
Logical exclusive OR between r1 and r2, storing the result in r1.  
Affected flags: ZF, SF

**XOR r1, imm**  
Logical exclusive OR between r1 and immediate value, storing the result in r1.  
Affected flags: ZF, SF

#### Arithmetic

**INC r1**  
Incremet r1.  
Affected flags: ZF, CF, OF, SF

**DEC r1**  
Decrement r1.  
Affected flags: ZF, CF, OF, SF

**ADD r1, r2**  
Add r1 and r2, storing the result in r1.  
Affected flags: ZF, CF, OF, SF

**ADD r1, imm**  
Add r1 and the immediate value, storing the result in r1.  
Affected flags: ZF, CF, OF, SF

**SUB r1, r2**  
Subtract r2 to r1, storing the result in r1.  
Affected flags: ZF, CF, OF, SF

**SUB r1, imm**  
Subtract the immediate value to r1, storing the result in r1.  
Affected flags: ZF, CF, OF, SF

**CMP r1, r2**  
Perform a SUB of r1 minus r2, without storing the result and only updating the
flags.  
Affected flags: ZF, CF, OF, SF

**CMP r1, imm**  
Perform a SUB of r1 minus the immediate value without storing the result and
only updating the flags.  
Affected flags: ZF, CF, OF, SF

#### Bit Operations

**SHL r1, n**  
Shift r1 n bits to the left (0 to 7), storing the result in r1.  
Affected flags: ZF, CF, SF

**SHR r1, n**  
Shift r1 n bits to the right (0 to 7), storing the result in r1.  
Affected flags: ZF, CF, SF

**ROL r1, n**  
Rotate r1 n bits to the left (0 to 7), storing the result in r1.  
Affected flags: ZF, CF, SF

**ROR r1, n**  
Rotate r1 n bits to the right (0 to 7), storing the result in r1.  
Affected flags: ZF, CF, SF

**BIT r1, n**  
Test bit n (0 to 7) or r1, setting ZF if it is set or clearing it otherwise.
Affected flags: ZF

#### Jump and Call

**JMP addr**  
Set the PC to the 16-bit address.

**CALL addr**  
Store the next PC in the stack (16-bit address) and sets the PC to the 16-bit
address.

**CALL [r1:r2]**  
Store the next PC in the stack (16-bit address) and sets te PC to the 16-bit
address provided by r1:r2.

**RET**  
Return from a call by setting PC to the top 16-bit value popped from the stack.

#### Branching

**BZ inst**  
Branch if Zero, will run the next instruction if ZF is set.  
Affected flags: BF

**BNZ inst**  
Branch if not Zero, will run the next instruction if ZF flag is not set.  
Affected flags: BF

**BC inst**  
Branch if Carry, will run the next instruction if CF flag is set.  
Affected flags: BF

**BNC inst**  
Branch if not Carry, will run the next instruction if CF flag is not set.  
Affected flags: BF

**BO inst**  
Branch if Overflow, will run the next instruction if OF flag is set.  
Affected flags: BF

**BNO inst**  
Branch if not Overflow, will run the next instruction if OF flag is not set.  
Affected flags: BF

**BS inst**  
Branch if Sign, will run the next instruction if SF flag is set.  
Affected flags: BF

**BNS inst**  
Branch if not Sign, will run the next instruction if SF flag is set.  
Affected flags: BF

**BI inst**  
Branch if Interrupt, will run the next instruction if IF flag is set.  
Affected flags: BF

**BNI inst**  
Branch if not Interrupt, will run the next instruction if IF flag is not set.  
Affected flags: BF

#### IO, flags and Misc

**HALT**  
Stop the execution until there is frame interrupt. If the interruption flag is
set, this will hang the CPU.

**PORT r1, r2**  
Write the value of r2 in the port number provided by r1. If there is an output,
the value will be stored in r1.

**NOP**  
No instruction has no effect.

**SIF**  
Set IF, disabling the interrupt.  
Affected flags: IF

**CIF**  
Clear IF, enabling the interrupt. If called in an interrupt handler, it won't
have effect. Use IRET instead to return from the interrupt.  
Affected flags: IF

**CCF**  
Clear carry flag.  
Affected flags: CF

**SCF**  
Set carry flag.  
Affected flags: CF

**COF**  
Clear overflow flag.  
Affected flags: OF

**IRET**  
Return from an interrupt handler by restoring F and PC from the stack.  
Affected flags: IF

### TR8 file format

It is a binary file to be run on the TR-8.

It doesn't have a header and it will be loaded at `0x0000` address. The
execution will start on that address with interruptions disabled (interruption
flag set).

See the memory map for further information.

## Author and Licence

This was made by [Juan J. Martinez](https://www.usebox.net/jjm/about/me/).

The code is MIT licensed.

Project website: https://git.usebox.net/tr8vm/about/