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So GROM is money-wise quite comparable - but got not only the advantage of (possibly) larger programs, but also an inherent copy protection, as G-EPROMs weren't available and replacement circuits using regular memory rather expensive. After all, when it comes to cartridge based applications, we do talk large numbers produced. Some just used the ROM space, maybe with bank switching, others used GROM.
How to use ti 99 emulator code#
Just fetch the byte.Īnd since GROMs where meant to hold code that gets interpreted by some low level code (nowadays called a bytecode interpreter), GROMs offered as well an incredible amount of possible online ROM, as well as faster access.Īpplication used a combination of either. For the TI 99/4 the streaming system takes care of that. For example, the basic fetch loop of an interpreter means just accessing the port and everything else is done by hardware, while with a conventional setup the CPU must fetch a pointer, address the byte in question, increment the pointer and store it again. When accessing some memory like Basic program, Video memory or, well, GROMs, it's always that these 'channels' have integrated address pointers auto-incrementing with each read or write access.
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The whole TI 99/4 is build around what today would be called a streaming architecture. But there is no cartridge known to use more than two pages for a total of 12 KiB ROM (*5) It's notable that TI also defined a mechanic for cards to do bank switching in this area, by swapping the upper 4 KiB as pages. In contrast, a regular ROM could only occupy a maximum of 8 KiB within the CPU address space. Pretty forward thinking for a 1979 machine.īeside, noone keeps any developer from adding GROM at additional address pairs, resulting in an almost unlimited amount of ROMs kept online. So even with the limitation of available chips and reserved address, still almost half a megabyte (480 KiB) could be added to a standard 99/4. Or more correct 640 KiB (*3), as certain addresses where reserved for OS ROMs, so each address pair could only address up to 40 KiB (*4). So why not turn this around from a cumbersome calling mechanic into a benefit? The TI 99/4's system also checks 16 such address pairs by default for GROMs, making a whopping 1 MiB of application ROM available. One such address pair can support 64 KiB of ROM, either as a single GROM or several in parallel (*1).Ħ4 KiB is already a ROM size hard to manage in a 16-bit address space without banking(*2). Even further, multiple GROM can share this address pair (usually 8). Most important reason would be address space.Ī GROM uses only two byte size addresses within the CPU address space. What was the purpose of this graphics ROM and why was there a need for two types of ROM? In other words, why not just have one ROM type like everyone else? Mainly to make GROM content loadable, so existing GROM application could be copied. While the mechanics (standard) was defined by TI, it wasn't until way later that third-party developers added GRAM. There was also GRAM, if memory serves, that would be Graphics RAM. More due the fact, that its primary usage within the TI99/4 was to hold GPL code - which itself stands for Graphic Programming Language. Jup, although, it wasn't really related to graphics at all. However, the TI cartridges could also contain a different type of ROM called "GROM".