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2ebdd254b3
This is something of a "Work in Progress" checkpoint of several features that are all half baked: 1. Allow loading of 16KB ROM files at address $C000 at run-time, not just at startup. See the "Load ROM..." File menu item. 2. Introduces the notion of "CPU Behaviors", so the core 6502 CPU implementation can match the behavior of either an early NMOS 6502, late NMOS 6502, or CMOS 65C02. Very little of this is actually implemented so far. 3. Adds a completely bogus implementation of the 6522 VIA (it does absolutely nothing right now). 4. Changes the address of the ACIA in the simulated system to match a real hardware implementation I put together.
153 lines
3.5 KiB
NASM
153 lines
3.5 KiB
NASM
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; minimal monitor for EhBASIC and 6502 simulator V1.05
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; To run EhBASIC on the simulator load and assemble [F7] this file, start the simulator
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; running [F6] then start the code with the RESET [CTRL][SHIFT]R. Just selecting RUN
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; will do nothing, you'll still have to do a reset to run the code.
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.feature labels_without_colons
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.include "basic.asm"
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; put the IRQ and MNI code in RAM so that it can be changed
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IRQ_vec = VEC_SV+2 ; IRQ code vector
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NMI_vec = IRQ_vec+$0A ; NMI code vector
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; setup for the 6502 simulator environment
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IO_AREA = $8800
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ACIAdata = IO_AREA ; simulated ACIA r/w port
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ACIAstatus = IO_AREA+1
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ACIAcommand = IO_AREA+2
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ACIAcontrol = IO_AREA+3
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; now the code. all this does is set up the vectors and interrupt code
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; and wait for the user to select [C]old or [W]arm start. nothing else
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; fits in less than 128 bytes
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.segment "MONITOR"
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.org $FF00 ; pretend this is in a 1/8K ROM
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; reset vector points here
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RES_vec
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CLD ; clear decimal mode
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LDX #$FF ; empty stack
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TXS ; set the stack
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; Initialize the ACIA
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ACIA_init
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LDA #$09
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STA ACIAcommand
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LDA #$1A ; Set output for 8-N-1 2400
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STA ACIAcontrol
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; set up vectors and interrupt code, copy them to page 2
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LDY #END_CODE-LAB_vec ; set index/count
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LAB_stlp
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LDA LAB_vec-1,Y ; get byte from interrupt code
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STA VEC_IN-1,Y ; save to RAM
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DEY ; decrement index/count
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BNE LAB_stlp ; loop if more to do
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; now do the signon message, Y = $00 here
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LAB_signon
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LDA LAB_mess,Y ; get byte from sign on message
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BEQ LAB_nokey ; exit loop if done
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JSR V_OUTP ; output character
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INY ; increment index
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BNE LAB_signon ; loop, branch always
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LAB_nokey
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JSR V_INPT ; call scan input device
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BCC LAB_nokey ; loop if no key
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AND #$DF ; mask xx0x xxxx, ensure upper case
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CMP #'W' ; compare with [W]arm start
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BEQ LAB_dowarm ; branch if [W]arm start
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CMP #'C' ; compare with [C]old start
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BNE RES_vec ; loop if not [C]old start
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JMP LAB_COLD ; do EhBASIC cold start
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LAB_dowarm
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JMP LAB_WARM ; do EhBASIC warm start
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; byte out to simulated ACIA
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ACIAout
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PHA ; save accumulator
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ACIAout_w
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LDA ACIAstatus ; Read 6551 status
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AND #$10 ; Is tx buffer full?
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BEQ ACIAout_w ; if not, loop back
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PLA ; Otherwise, restore accumulator
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STA ACIAdata ; write byte to 6551
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RTS
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; byte in from simulated ACIA
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ACIAin
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LDA ACIAstatus ; Read 6551 status
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AND #$08 ;
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BEQ LAB_nobyw ; If rx buffer empty, no byte
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LDA ACIAdata ; Read byte from 6551
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SEC ; Flag byte received
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RTS
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LAB_nobyw
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CLC ; flag no byte received
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no_load ; empty load vector for EhBASIC
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no_save ; empty save vector for EhBASIC
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RTS
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; vector tables
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LAB_vec
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.word ACIAin ; byte in from simulated ACIA
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.word ACIAout ; byte out to simulated ACIA
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.word no_load ; null load vector for EhBASIC
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.word no_save ; null save vector for EhBASIC
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; EhBASIC IRQ support
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IRQ_CODE
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PHA ; save A
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LDA IrqBase ; get the IRQ flag byte
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LSR ; shift the set b7 to b6, and on down ...
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ORA IrqBase ; OR the original back in
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STA IrqBase ; save the new IRQ flag byte
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PLA ; restore A
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RTI
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; EhBASIC NMI support
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NMI_CODE
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PHA ; save A
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LDA NmiBase ; get the NMI flag byte
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LSR ; shift the set b7 to b6, and on down ...
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ORA NmiBase ; OR the original back in
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STA NmiBase ; save the new NMI flag byte
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PLA ; restore A
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RTI
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END_CODE
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LAB_mess
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.byte $0D,$0A,"6502 EhBASIC [C]old/[W]arm ?",$00
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; sign on string
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; system vectors
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.segment "VECTORS"
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.org $FFFA
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.word 0 ; NMI vector
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.word RES_vec ; RESET vector
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.word 0 ; IRQ vector
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