Bryan Parkoff wrote:
"Michael J. Mahon" <mjmahon@aol.com> wrote in message yPudnVdjOalvU1bYnZ2dnUVZ_tGsnZ2d@comcast.com">news:yPudnVdjOalvU1bYnZ2dnUVZ_tGsnZ2d@comcast.com...Bryan Parkoff wrote:I notice that table translation for Disk II has 81 valid out of 256 ASCII codes, but 64 values are used only. Here is 17 unused values below.92, 93, 94, 95, 99, 9C, A4, A5, A9, AA, C9, CA, CC, D2, D4, D5, & E4Why didn't Disk II use 17 values for data buffer? I am aware which values are used for address field and data field.As Alex pointed out, powers of two are what matter. But these codes (except for D5 and AA, which appear only in headers) are not used because they do not meet the requirement for *only* one pair of consecutive 0 bits and *at least* one pair of consecutive 1 bits in bits 0-6.In addition a very short piece of code in Disk][ ROM builds the table of valid bytes.It makes sense.Relaxing the "at least one pair of 1 bits" rule would allow 95, A5, CA, and D4 as additional "delimiters". In practice, the Disk ][ Controller can reliably read other nibbles as well, but not enough to allow 128 different nibbles, since that would require *all* combinations of bits 0-6.Do you allow A9, D2 and D5 like you said 95, A5, and CA? Does it mean that 8 bit value can't have two pair of zero bits? It can only have one zero bit and pair of two zero bit combination in any 8 bit value.
You're correct, these three nibbles also are only excluded because they don't have a pair of 1's in bits 0-6. They would be read correctly by the disk controller. -michael NadaNet networking for Apple II computers! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it's seriously underused."