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135 lines
3.4 KiB
NASM
135 lines
3.4 KiB
NASM
; acia
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;
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; Manage I/O from an asynchronous communication interface adapter (ACIA).
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; provides "aciaPutC" to put c char on the ACIA as well as an input buffer.
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; You have to call "aciaInt" on interrupt for this module to work well.
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;
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; "aciaInit" also has to be called on boot, but it doesn't call "ei" and "im 1",
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; which is the responsibility of the main asm file, but is needed.
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; *** DEFINES ***
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; ACIA_CTL: IO port for the ACIA's control registers
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; ACIA_IO: IO port for the ACIA's data registers
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; ACIA_RAMSTART: Address at which ACIA-related variables should be stored in
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; RAM.
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; *** CONSTS ***
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; size of the input buffer. If our input goes over this size, we start losing
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; data.
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ACIA_BUFSIZE .equ 0x20
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; *** VARIABLES ***
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; Our input buffer starts there. This is a circular buffer.
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ACIA_BUF .equ ACIA_RAMSTART
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; The "read" index of the circular buffer. It points to where the next char
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; should be read. If rd == wr, the buffer is empty. Not touched by the
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; interrupt.
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ACIA_BUFRDIDX .equ ACIA_BUF+ACIA_BUFSIZE
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; The "write" index of the circular buffer. Points to where the next char
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; should be written. Should only be touched by the interrupt. if wr == rd-1,
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; the interrupt will *not* write in the buffer until some space has been freed.
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ACIA_BUFWRIDX .equ ACIA_BUFRDIDX+1
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ACIA_RAMEND .equ ACIA_BUFWRIDX+1
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aciaInit:
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; initialize variables
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xor a
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ld (ACIA_BUFRDIDX), a ; starts at 0
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ld (ACIA_BUFWRIDX), a
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; setup ACIA
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; CR7 (1) - Receive Interrupt enabled
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; CR6:5 (00) - RTS low, transmit interrupt disabled.
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; CR4:2 (101) - 8 bits + 1 stop bit
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; CR1:0 (10) - Counter divide: 64
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ld a, 0b10010110
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out (ACIA_CTL), a
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ret
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; Increase the circular buffer index in A, properly considering overflow.
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; returns value in A.
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aciaIncIndex:
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inc a
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cp ACIA_BUFSIZE
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ret nz ; not equal? nothing to do
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; equal? reset
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xor a
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ret
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; read char in the ACIA and put it in the read buffer
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aciaInt:
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push af
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push hl
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; Read our character from ACIA into our BUFIDX
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in a, (ACIA_CTL)
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bit 0, a ; is our ACIA rcv buffer full?
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jr z, .end ; no? a interrupt was triggered for nothing.
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; Load both read and write indexes so we can compare them. To do so, we
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; perform a "fake" read increase and see if it brings it to the same
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; value as the write index.
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ld a, (ACIA_BUFRDIDX)
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call aciaIncIndex
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ld l, a
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ld a, (ACIA_BUFWRIDX)
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cp l
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jr z, .end ; Equal? buffer is full
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; Alrighty, buffer not full. let's write.
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ld de, ACIA_BUF
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; A already contains our write index, add it to DE
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call addDE
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; increase our buf ptr while we still have it in A
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call aciaIncIndex
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ld (ACIA_BUFWRIDX), a
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; And finally, fetch the value and write it.
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in a, (ACIA_IO)
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ld (de), a
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.end:
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pop hl
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pop af
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ei
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reti
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; Read a character from the input buffer. If the buffer is empty, loop until
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; there something to fetch. Returns value in A.
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aciaGetC:
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push de
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.loop:
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ld a, (ACIA_BUFWRIDX)
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ld e, a
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ld a, (ACIA_BUFRDIDX)
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cp e
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jr z, .loop ; equal? buffer empty, wait.
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; Alrighty, buffer not empty. let's read.
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ld de, ACIA_BUF
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; A already contains our read index, add it to DE
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call addDE
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; increase our buf ptr while we still have it in A
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call aciaIncIndex
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ld (ACIA_BUFRDIDX), a
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; And finally, fetch the value.
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ld a, (de)
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pop de
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ret
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; spits character in A in port SER_OUT
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aciaPutC:
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push af
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.stwait:
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in a, (ACIA_CTL) ; get status byte from SER
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bit 1, a ; are we still transmitting?
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jr z, .stwait ; if yes, wait until we aren't
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pop af
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out (ACIA_IO), a ; push current char
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ret
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