Lesson 31 · FX29 - point at a font digit

FX29 - point at a font digit

To draw a number, a program points the index register at that digit's built-in font sprite. Today you implement FX29, which computes the font address from a register.

The goal

Implement FX29 so it sets I to the address of the font sprite for the digit in VX.

Start here - the target

TO DO

Scenario: FX29 aims the index register at a font sprite

Giventhe font is loaded at 0x50, V0 holds 0x02, about to execute 0xF029

WhenStep runs it

ThenI becomes 0x5A (0x50 + 2*5, the start of the digit-2 sprite) and the byte at I is 0xF0 (the first row of the 2 sprite)

Andwith V0 = 0x00, FX29 sets I to 0x50 (the digit-0 sprite)

Background

FX29 connects the register file to the font you loaded back in lesson 5. It takes the digit in VX and sets I to the memory address of that digit’s 5-byte font sprite, so a following DXYN can draw it. The calculation is exactly the layout you pinned when loading the font: the digits sit back to back at 0x50, five bytes each, so the sprite for digit d begins at 0x50 + d*5. Digit 2 is at 0x5A, digit 0 at 0x50.

This is the standard way a CHIP-8 program shows a score or a counter: put the digit’s value in a register, FX29 to aim I, then DXYN with a height of 5 to stamp it on screen. The spec checks not just the computed address but that the byte living there is the expected first row of the sprite (0xF0), which confirms the font really was loaded where this opcode assumes. The capstone will use exactly this pairing to draw a digit.

Make it work

const FontStart = 0x50
// in the 0xF000 arm:
case 0x29:
  // each font digit is 5 bytes; point I at the sprite for the
  // low nibble of VX
  v.i = FontStart + uint16(v.V[x])*5
  return nil

CheckpointDONE

FX29 points I at the built-in font sprite for a digit. Commit and stop here.