Emulating the
BBC Micro
in Javascript
Matt Godbolt
Emulating the
BBC Micro
in Javascript
Matt Godbolt
Trying to recapture a lost youth
What's a BBC Micro?
Why BBC Micro?
Why BBC Micro?
Why Javascript?
- Surely it'd be too slow?
- "Worse" than C++?
- Why not?
- Runs in browser
The 6502
6502 Computers
6502 101
Registers
- 8-bit
Accumulator - 8-bit
X,Yindex Processor statusStack pointer- Program counter
Instructions
LDA STA LDX ... ; load / store A, X, Y
TAX TXA ... ; transfer A to X
PHA PLA ; push A / pull A
CMP ; compare with A
ADC SBC ; add / subtract with carry
CLC SEC ; clear / set carry
JMP ; jump
BEQ BNE BCS BCC ; branch ==, !=, carry set/clear
BMI BPL ... ; <0, >=0
JSR ; jump to subroutine
RTS ; return from subroutineAddressing modes
a9 20 LDA #$20 ; A = 0x20
a5 70 LDA $70 ; A = readmem(0x70)
ad 34 12 LDA $1234 ; A = readmem(0x1234)Addressing modes
bd 34 12 LDA $1234, X ; A = readmem(0x1234 + X)
b9 34 12 LDA $1234, Y ; A = readmem(0x1234 + Y)
b1 70 LDA ($70), Y ; t1 = readmem(0x70)
; t2 = readmem(0x71)
; addr = t1 | (t2<<8)
; A = readmem(addr + Y)
b5 70 LDA $70, X ; A = readmem(0x70 + X)Emulating a 6502
- Fetch next instruction
- Decode it
- Execute it
Emulation
var a = 0, x = 0, y = 0, pc = readword(0xfffe);
while (true) {
switch (readmem(pc++)) {
case 0xa9: // LDA #imm
a = readmem(pc++); break;
case 0xad: // LDA $addr
var addr = readword(pc); pc += 2;
a = readmem(addr);
break;
// and so on for all other instructions...
}
}Except...
- Setting flags
- Memory access
- Hardware
- Interrupts
- Handling time
Processor status: Flags
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|
| Negative | oVerflow | - | - | Decimal | Interrupt disable |
Zero | Carry |
- Z & N set on ALU/load instruction
- C set by shifts & arithmetic
- V set by arithmetic
- D, V, C and I controlled by special instructions
So now we have:
case 0xa9: // LDA #imm
a = readmem(pc++);
p.z = !a;
p.n = !!(a & 0x80);
break; Memory
| 0x10000 | |
|---|---|
| 0xff00 | 256 bytes OS ROM |
| 0xfe00 | 256 bytes hardware |
| 0xfd00 | 256 bytes ROM |
| 0xfc00 | 256 bytes hardware |
| ↑ ↑ ↑ 0xc000 |
15.5KB OS ROM |
| ↑ ↑ ↑ ↑ 0x8000 |
16KB Paged ROM |
| ↑ ↑ ↑ ↑ 0x4000 |
16KB RAM |
| ↑ ↑ ↑ ↑ 0x0000 |
16KB RAM |
var ram = new Uint8Array(0x8000), romsel = 0, roms = [];
var os = load("OS");
roms[15] = load("BASIC");
function readmem(addr) {
if (addr < 0x8000) return ram[addr];
if (addr < 0xc000) return roms[romsel][addr - 0x8000];
if ((addr >= 0xfe00 && addr < 0xff00)
|| (addr >= 0xfc00 && addr < 0xfd00))
return readhw(addr);
return os[addr - 0xc000];
}function writemem(addr, b) {
if (addr < 0x8000) ram[addr] = b;
if (addr >= 0xfe00 && addr < 0xff00) writehw(addr, b);
// else does nothing - it's ROM
}The outside world
- Video chip:
- Pixel generator reads from RAM
- Simple character-mapped mode
- Bitmap modes with 2, 4, 16 colours
- Sound (SN76489)
- Timers (6522 VIA)
Demo
What happened?
Fixing it
case 0xa9: // LDA #imm
a = readmem(pc++);
p.z = !a;
p.n = !!(a & 0x80);
video.run(2);
timers.run(2);
break;Demo
More games
Zalaga
Inside the 6502
| $84 10000100 | STY zp |
|---|---|
| $85 10000101 | STA zp |
| $86 10000110 | STX zp |
| $87 10000111 | ??? |
Real decoding
©2010 Visual6502.org, used with permission
Inside the 6502
+-----+-----+-----+ | A | X | Y | +-----+-----+-----+ | | | /+--------- | --- | (!s && !t) || | | | +----------+ | | | | opcode|| | | | | | 100001st |------->t | | +----------+ | | | ^ \---------- | --->s | | \ | / | \ | / select write bus store
Inside the 6502
+-----+-----+-----+ | A | X | Y | +-----+-----+-----+ | | | /+--------- | --- | (!s && !t) || | | | +----------+ | | | | opcode|| | | | | | 100001st |------->t | | +----------+ | | | ^ \---------- | --->s | | \ | / | \ | / select write bus store
Inside the 6502
| $84 10000100 | STY zp |
|---|---|
| $85 10000101 | STA zp |
| $86 10000110 | STX zp |
| $87 10000111 | SAX zp |
Even more games
An admission
Protection systems
- Decrypt game code
- Keys include:
- Magic values loaded from tape (or disc)
- Decryption code itself
- Hardware timers & registers
- Interrupt information
- Read-modify-write
Accurate timing information
Visual 6502
ROL $fe48
| Cycle | Read / Write | Address | Description |
|---|---|---|---|
| 0 | Read | 0d2d | Read opcode (2e) |
| 1 | Read | 0d2e | Read low byte of address (48) |
| 2 | Read | 0d2f | Read high byte of address (fe) |
| 3 | Read | fe48 | Read memory at $fe48 |
| 4 | Write | fe48 | Do rotate — writes unmodified value! |
| 5 | Write | fe48 | Store to $fe48 |
Fixing it
case 0x2e: // ROL abs
addr = readword(pc); pc += 2;
hardware.run(3);
val = readbyte(addr);
hardware.run(1);
writebyte(addr, val); // unmodified value
hardware.run(1);
val = (val << 1) | (carry ? 1);
carry = val & 0x100;
writebyte(addr, val & 0xff); // modified value
hardware.run(1);
break;Cycle stretching
Conclusion
Resources
Some Demos
Emulating theBBC Microin Javascript
Matt Godbolt