Something like this:
PUSH H ; Save HL from BASIC
CALL HH ; Call HideyHole Routine
LXI H, 2020H ; First JMP Address to Update
MVI A, 10H; offset to accumulator
ADD E ; add CALL address to offset
MOV M, A ; save lower byte new address
INX H
MVI A, 0 ; get rdy to add carry bit
ACI D ; add carry bit
MOV M, A ; save upper byte new address
; Repeat for as many jump addresses
; that you have in the remainder
POP H ; retrieve original HL value
; Rest of program goes here
; stored in your hidey hole
HH: DAD SP
MOV E,L
MOV D,H
RET
On Tue, Mar 7, 2023 at 10:16 AM [email protected] <[email protected]>
wrote:
> Thanks for the info Ken.
>
> That does seem more expensive as you will be executing the relative jump
> routine every time you want to JMP. In my case, I will likely be using JMP
> as a way to loop. so it would likely impact performance.
>
> My initial thought was rather than do a relative jump in the program
> itself, have the first step of the program modify all of the JMP addresses
> used in that program specifically. Is that something you have tried at all?
>
> -George
>
> On Tue, Mar 7, 2023 at 9:42 AM Kenneth Pettit <[email protected]> wrote:
>
>> Hi George,
>>
>> Yes, using CALL is the way to perform relative jumps in your ML
>> program. The idea is this:
>>
>> Poke a helper function into a hidey hole (on M100 there is a nice 52
>> byte location you can use for this that I will explain). So the way to
>> do a relative jump is to add your jump distance as a data byte after the
>> CALL:
>>
>> CALL myRelJump ; Relative jump the distance in the next two
>> bytes (or one if you want)
>> DW 0xFFE5 ; Jump -27 (signed jump)
>>
>> Then at myRelJump you would do something like (not tested, you will have
>> to play with it):
>>
>> PUSH H ; Save HL. Also modifies SP
>> PUSH D ; Save DE.
>> PUSH B
>> LXI H,FFFAH ; Load HL with -8
>> DAD SP ; HL now has the Stack location of the return address
>> (i.e. points to your relative jump distance)
>> MOV E,L ; Save stack location in DE
>> MOV D,H
>> LHLX ; Undocumented opcode to load HL from (DE) ... HL now
>> points to your ML program
>> MOV C,M ; Get LSB of the relative jump distance
>> INX H ; Point to MSB
>> MOV B,M ; Get MSB of relative jump distance
>> DAD B ; Add the offset to the PC location (in HL) of the
>> program
>> SHLX ; Undocumentd opcode to save HL at (DE) ... save new
>> return address to stack
>> POP B
>> POP D
>> POP H
>> RET ; This now returns to the new location you just
>> calculated
>>
>> Note that is will be SLOWER ... you are executing 17 opcodes for each
>> jump instead of one!!
>>
>> The hidey hole I was talking about is an array used only by the TEXT
>> application. It saves 26 2-byte pointers (one for each line of the LCD
>> / DVI, plus an extra containing 0xFFFF) to indicate the location in the
>> edited .DO file of the start of that line. I call it the "line starts
>> array". When runing BASIC, this memory region is unused and can be
>> poked with any ML you want. On M100, this is at address F6EBH.
>>
>> Hope this helps. Again, this is UNTESTED code, but should give you the
>> idea.
>> Ken
>>
>> On 3/7/23 4:44 AM, [email protected] wrote:
>> > Hi all,
>> >
>> > One of the things I struggled with in trying to build 8085 subroutines
>> > for BASIC program was the lack of a relative jump command.
>> >
>> > So far, the approach I have taken to embed ML programs into BASIC has
>> > been to poke them into arrays, thus allowing BASIC to manage the
>> > routines within memory. However, I then have to use BASIC to get
>> > VARPTR of the array and update all JMP locations before calling the
>> > subroutine, just in case BASIC has moved the array.
>> >
>> > The other thought I had was, can I have the ML program modify itself
>> > when it runs? I was racking my brain reading the Inten 8085
>> > documentation, and as far as I can tell there is no easy way to read
>> > the program counter into any of the register pairs, which I would want
>> > to do in order to calculate offsets and update JMP locations.
>> >
>> > However I had one thought. It seems that both CALL and RST
>> > instructions push the program counter onto the stack. Is it possible
>> > to leverage either of these and then RET to return to my program, and
>> > then immediately decrement the stack pointer and to retrieve the
>> > previous value.
>> >
>> > Then I can use that value to calculate all the offsets and update the
>> > JMP locations.
>> >
>> > Does anyone have any constructive input here?
>> >
>> > -George
>>
>>
