|OUT (n), A||Sends a byte to port n via the accumulator.|
|OUT (C), reg||Sends a byte to port C via register reg.|
|IN A, (n)||Receives a byte from port n via the accumulator. Does not affect flags.|
|IN reg, (C)||Receives a byte from port C via register reg.|
A port is a device that lets the CPU transfer bytes between other pieces of otherwise unconnected hardware. What's that? You want an analogy? Okay, imagine you wanted to get a car from Yokohama to San Francisco; you couldn't just drive it over because they're separated by thousands of miles of open ocean. Instead, you'd take the car to the port of Yokohama, have a boat take it to the port in San Francisco, and drive off. Similarly, the CPU and the keypad have no real connection, so the port is used to interact.
The keyport on the TI-83 Plus is port #1, so we replace n in the two instructions with 1.
The first thing to do is enable the key group we want to read from. This is done by sending the value of that group to the key port. We then read from the key port and check the value returned. You can get the values from this table.
|EF||0||1||4||7||,||SIN||APPS||X, T, θ, n|
ReadKey: LD A, %11111101 ; Check for [-] OUT (1), A IN A, (1) CP %11111011 JP Z, Minus LD A, %11111110 ; Check for [up] OUT (1), A IN A, (1) CP %11110111 JP Z, Up JP ReadKey Minus: LD HL, zs_minus b_call(_PutS) RET Up: LD HL, zs_up b_call(_PutS) RET zs_up: .DB "You pressed UP !", 0 zs_minus: .DB "You pressed - !", 0
Loop: LD A, %11011111 ; Enable group OUT (1), A IN A, (1) ; Input a key CP %11110111 ; Check if it's [LOG] RET Z ; End if so JP Loop
In Program 22-2 then, the key port was sending the bitwise AND of [LOG] and [ALPHA] which is %01110111. CP didn't work because it was looking for the exact value %11110111. BIT, on the other hand, will work because bit 3 is still zero.
If you want the calcultor to do something when a key is pressed, regardless of whether any other keys in the group are pressed, you should use BIT (or a shift instruction if possible). However, if you wanted a different action to be taken when two or more keys are pressed down, then you'd have to either use CP, or have a kind of a BIT chain.
b_call(_RunIndicOff) LD HL, $1C23 LD (x_pos), HL DispText: b_call(_ClrLCDFull) LD HL, (x_pos) LD (PenCol), HL LD HL, string b_call(_VPutS) LD C, 1 InKey: LD A, %10111111 ; Check for [DEL] to exit OUT (C), A IN A, (C) BIT 7, A JR NZ, InArrow LD A, $FF ; Reset key port OUT (C), A RET InArrow: LD A, $FF ; Reset key port OUT (C), A LD A, %11111110 OUT (C), A IN B, (C) BIT 0, B JP Z, Down BIT 1, B JP Z, Left BIT 2, B JP Z, Right BIT 3, B JP Z, Up JP InKey Down: CALL MoveDown BIT 1, B CALL Z, MoveLeft BIT 2, B CALL Z, MoveRight JP DispText Left: ;There is no need to check for Down key anymore. CALL MoveLeft BIT 3, B CALL Z, MoveUp JP DispText Right: CALL MoveRight BIT 3, B CALL Z, MoveUp JP DispText Up: CALL MoveUp JP DispText MoveDown: LD A, (y_pos) ; Check if at bottom edge of screen CP 57 RET Z INC A ; Down one pixel LD (y_pos), A RET MoveUp: LD A, (y_pos) ; Check if at top edge of screen OR A RET Z DEC A ; Up one pixel LD (y_pos), A RET MoveLeft: LD A, (x_pos) ; Check if at left edge of screen OR A RET Z DEC A ; Left one pixel LD (x_pos), A RET MoveRight: LD A, (x_pos) ; Check if at right edge of screen CP 96-28 ; 96 - number of pixels the string takes up RET Z INC A ; Right one pixel LD (x_pos), A RET x_pos: .DB 0 y_pos: .DB 0 string: .DB "Let\'s Go!", 0