for hw expert designers, a Frankestein GFX CARD for msx

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By PingPong

Prophet (3623)

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20-01-2021, 20:26

How about a Video Cart Adapter based on C64 VIC-II on board, like is playsonic with sid?
Could the VIC-II driven in a Specific MSX Slot (The VIC itself can only address 16K, but there is a kind of MMU in C64 that allows to do paging of VIC-II RAM

Would it possible due to heavy RAM BandWidth usage without eccessive z80 speed penalty?
May be performances could be greater than c64's 6502 cpu.

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By rolandve

Master (233)

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20-01-2021, 21:23

C64 runs at 1 MHZ, so MSX at 3.xx should be sufficiently fast. I think you'll run in to very nice challenges from an electrical/logical point of view. Why not the 8564/8566 VIC-IIe? This is the chip of the C128 and it was designed to work with a Z80 because the C128 could run CP/M. It also has a C64 compatibility mode.

By PingPong

Prophet (3623)

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20-01-2021, 21:50

the VIC-IIe is basically the VIC with 2 registers added. non gfx feature added and the ability to drive the c128 cpu at 2mhz but with screen blanked, so where is the gain?

By PingPong

Prophet (3623)

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21-01-2021, 15:50

i've seen that the VIC-II issues a busrq on the main CPU, and this could be a problem. maybe having some kind of fast ram and doing some time sharing (and buffering) could allow simultaneus access of both CPU/VIC like it happens with the ADVRAM ?

By PingPong

Prophet (3623)

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22-01-2021, 19:43

No one can tell something about the possibility to make a similar hw?
To me it is possible to do but at the cost of recreating the two phase mechanism usually typical in 6502 systems.
But this would be a z80 stopped for half time...

By Lynn

Rookie (32)

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23-01-2021, 12:37

rolandve wrote:

C64 runs at 1 MHZ, so MSX at 3.xx should be sufficiently fast.

You can't compare clock speed between different architectures like that. They have different instruction sets that don't do exactly the same thing in the same amount of cycles. a Z80 at 3MHz isn't necessarily 3 times as fast as a 6510 at 1MHz.

By rolandve

Master (233)

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23-01-2021, 15:41

Correct. Problem is not even the instruction architecture, its the complete architecture with memory access, VDP access for MSX etc. This makes the the raw MHZ perhaps the only non architecture dependent performance indicator. Another stupid test both C64 and MSX can do is: write a basic program that displays all numbers to 65000 on the screen. The first over the finish wins, because this test simply tests the performance of the complete operational systeem.

By Grauw

Ascended (9579)

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23-01-2021, 16:25

Yeah the architecture can be an important factor, especially if RAM is shared with the VDP, this introduces wait cycles to the CPU.

What also matters is how many cycles it takes to execute instructions, on Z80 instructions take 4-23 cycles (5-25 on MSX), whereas on the 6502 they take 2-7. But the Z80 has more registers and complex instructions so it needs less instructions to complete a task. All of this makes it difficult to compare.

For a BASIC speed test, an additional factor is the efficiency of the BASIC interpreter implementation, this varies wildly and makes it difficult to perform raw CPU speed tests with it. But in practice (in assembly), I think probably an MSX and C64 are similar in terms of processing speed. At least I’ve not really seen strong claims that the one is significantly faster than the other.

By PingPong

Prophet (3623)

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23-01-2021, 20:01

here the problem is not the speed. but the architecture.
6502 had a fairly simple memory access schema, for example for 1/2 of the clock period one can assume that the cpu will not do memory access so you can reserve some fixed bandwidth for cpu and some for video.

However z80 has a variable length memory cycle making difficult sync video with cpu.
maybe one can use a very fast ram with double clock rate able to reserve 1/2 of the band to cpu and the other to vicdeo?

By rolandve

Master (233)

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23-01-2021, 21:20

If the goal is to experience the C64, buying one is way better: you also get a very good sound chip with it. If you just want the looks, perhaps changing the color palet will do. If its for the challenge: I think this will be a big one.

By PingPong

Prophet (3623)

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23-01-2021, 21:46

hi, it is not for experience of course, instead to see what is is possible with such hw paired with a z80 cpu.
And of course i need some more hint about tech issues, i already know it is not a easy task

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